Currently running a 3.91 gear in a 8.25 rear behind a A-833 OD trans (1.00 ration 3rd/.67 ratio 4th) and mild 318 in my Duster. Driveshaft is 2.75” diameter. After the gear change last month, above 2300 rpm/65mph in 4th/OD, there was a minor vibration/resonance. Had new u joints put in and re-balanced. According to ship, the driveshaft is not perfectly true, but almost with very little runout. That improved the issue down to what I can describe as more of a medium frequency resonance that actually intensifies slightly as I let off of the gas, and not really a vibration. It’s is still there to a lesser extent when I pop it out of gear. Feel it more up in the front under my seat. Rear tires are new and balanced, same for front.

Motor mounts are about 8 months old and ever since, the engine has always seemed rougher when free revving in neutral with those mounts, even before the 3.91 gear and prior to that, the 4 speed swap.

Does this sound like pinion angle slight Imperfection, or driveshaft half critical speed situation, or maybe slightly worn axle bearing? I am still driving the car, because I don’t run it on the dragstrip and it isn’t shaking me to death, but I don’t want to risk the driveshaft exploding (if that is even a legitimate risk or fear).

The car is talking to you asking you to fix its problem
I would concentrate on fixing now before it hurts you with a parts failure
Can you jack the car up and put it on jack stands high enough for you or a helper crawl under the car while running? If so do that and have someone either drive the car on jackstands or have someone look at the drive shaft under power in gear where the car vibrates to help identify where the problems are
Many things can cause problems like your having, a tire that is out of round and so on
Have fun and let us know what you find, that could help others on here later

I'm wondering if the mts had a slightly different thickness & changed the ujoint angles a bit or if they are just stiffer. the noise got worse with the gear change so that would point me there but what confuses things a bit is the shaft balancing helped. I would check the angles & confirm that the front yoke moves smoothly by hand back & forth in both planes & the rear ujoint is smooth in its one plane. too many changes at the same time does complicate diagnosing. Keep us updated on any info (we'll get it)

Yes, the motor mounts do make me wonder, as they were done before the trans and gear swap when the car was a 904 with 3.21s. The trans swap came about a month after and the gear swap about a month ago.

I think the ujoints and balancing helped, mainly because the front joint was so utterly shot after 6 years of service since the last balance job and new ujoints.

The car sits on Mancini XHD +1 springs, with lowering blocks to bring rear down closer to level with the front.

& another potential (I had that once) is not enough front yoke engagement into the trans (& or bad ext housing bushing). Mine was a PT vibration at random sort of (mostly deccel) & I swapped the correct shaft in & all good.

attached is picture of the current ride height...

The snout on the front of the driveshaft is better than 3/4 engaged into the tailshaft and the snout is roughly 6" long. Before the 3.91 swap, I didn't feel anything off at the same highway speed. Maybe bc the engine revs were lower.

I have run up to the same speed in 3rd and that is about 3300 rpm. Couldn't really tell if it was there, probably because of the added harshness from the engine. I don't think the motor mounts are the softest.

If the tailshaft bushing in the trans has wear, should I be concerned until I can get the trans out and freshen it?

I ain't for sure & it would depend on how sloppy it is. How much end play does the driveshaft have? (when you butt it all the way into the trans how much distance is there to move it back till the rear ujoint is seated). excessive endplay is more likely to be a cause than a worn bushing (unless it is real bad) & right now I'm thinking the bushing ain't the problem (it might not be perfect but I am thinking elsewhere) however too much end play can definitely be it. I like ~3/4-7/8, enough to get the ujoint over the yoke when installing plus a bit more measured with springs compressed at ride height/weight (stands under axle). plus we have the angles and the pig as potentials. As Cab says we gotta stay after it.

With the driveshaft bolted into the rear yoke, there is not front to back play obviously. With the new front u-joint, you can maybe move the front of the drive shaft side to side a bit (haven’t measured, but I would guess no more than .250”. This is the snout of the driveshaft only, and not the u-joint flexing with it. As extra information, there isn’t any play on the rear pinion/yoke back at the differential.

I am a skeptic that, although the gear change increased driveshaft speed and it’s probably at or approaching half critical speed of a 2.750” shaft, that it’s a major player in what I am feeling.

And yes, just go to stay after it. I am more OCD than anything about so called issues like I am describing.

the front to back distance would be measured with the rear ujoint loose just like you were first installing the driveshaft (& that is a good time to check that) when you butt it all the way forward to clear the rear yoke then move it back to seat into the yoke, that distance. Right now the side to side slop the front yoke has in the extension housing bushing you mentioned, 1/4 seems like a ton or did I misread something.

That 1/4" guess is probably way more than it is, so that was misleading. I can push the snout side to side when installed, but only a very small amount.

There should not be any movement with the driveshaft installed in the car , no up or down and no side to side movement, EVER It should rotate smoothly in a straight plane, no wobbling
Wobbling(up or down or side to side) makes vibrations, straight and true do not
It sounds to me that maybe the front U joint tranny yoke is junk It should not move up or down or side to side when installed on the tranny output shaft
Get that good looking rascal up in the air on jack stands so you can run it in gear to find the cause

Yoke not far enough into the trans will do this. Sounds like he's currently about 2" from bottoming out, should be less than 1".

Driveshaft is too short.

Did you put poly mounts in the trans yoke as well?

I am not sure what you are referring to. You mean the trans mount? if so, it is just a regular mount, not a poly.

Well fellas the only different thing is the TRANS, so I'm thinking the tail shaft bushing or the bearing. I can't remember if the 904 and four speed use the same length shaft.

Mostly true. Used 3.21 gears for first few months with the A833 (which was not rebuilt) and didn't feel anything between 65-70 mph now, but maybe that has some to do with the driveshaft now spinning at a higher rate of speed that with the 3.21s.

Almost immediately after I left the diff shop for the gear change, I felt the resonance on the highway above 60mph. Two days later, the driveshaft was balanced and new Spicer u-joints were put on the front and rear since they were shot. That re-balance and new joints helped immensely, but as said, still not completely gone.

I should mention that this is the same driveshaft that I shortened to use behind the previous 904 when I swapped in the 8.25 rear. That was the first time the driveshaft was balanced and new joints were installed. The very same driveshaft was used when swapped over to the A833 trans, with ZERO modifications to the driveshaft done.

I will see if I can take some pictures of the driveshaft if I get home the office early enough tonight.

that's what Sherlock Holmes/Columbo and lieutenant Kenda would (all) call a major clue. Keep It simple Sam. Unless they messed up the shaft somehow during the swap (ujoint clips/caps can be delicate/problematic) I'd be thinking about swapping the pig at least for a test even with an open gearset. I know it is a pain.

Yes, I had thought of that, but, being that it is not an 8 3/4 with a removable third member and is a 8 1/4, that isn't exactly easy. The 3.21 setup previously had a Spicer limited slip added and functioned great. The 3.91 gears in it now were slightly used (about 2000 miles, but all teeth were in great shape). The same limited slip unit with 3000 miles on it was used. Very slight quiet whine from gears at any speed, but not louder on deceleration like what would occur with bad pinion crush sleeve.

What about an axle bearing being knackered a bit during the gear swap?

As mentioned, the problem really isn't a vibration more than it is a deep, almost cyclical hum that you can feel resonating mostly below the driver side seat and mid to front of drivetrain. Not to say its not felt further back as well.

I missed that it ain't an 8&3/4. do keep us updated.