Came in here for this. Have you checked your pinion angle? You can exacerbate other driveline vibrations if your pinion angle is bad.

Verifying that both yokes are parallel to each other today and if adjustments are needed.

Here you go boys. 1.5 inches exposed and the rest is engaged, which is 4.5 inches.

The drive shaft is now spinning about 1000 rpm faster with the 3.91 gear. The faster it spins, the harder it shakes.

The shaft looks too short coupled with the fact that the shaft has some runout according to your machinist equals vibration.

Kevin

Grabbing at straws here but I had one in a pickup years ago that did what your describing after having new U-joints installed...and old mopar guy told me to "Spin the driveshaft half a turn" take it loose and spin it half a turn and reinstall. It worked....scratch my head to this day over that one...
I'd say it's worth a try...

Measure the inside spline length
I like to see from .750 to 1.0 yoke extension out of the tranny tailshaft housing from bottomed out with the car on the ground loaded ready to drive You have way more than that on yours
The one picture of the front tranny yoke on the driveshaft, it looks like to me that the drive shaft is not centered in relation to the yoke center I'm sure it is not out of center due to if it was as bad as I see it you wouldn't be able to drive it over 20 MPH without the drive shaft breaking the output shaft and tail shaft housing on the tranny

Sorry about that, yes the mount. I had the same exact deal going on in my car and never got it figured out. Someone said it was the pinion angle. The addition of the poly mount didn't help either in my case, but it was not a DD so I lived with it.

Once pushed all of the way in and backed out 1 inch, there is about .750" of slip yoke exposed, versus about 1.5" installed. There is currently 4.5 inches of slip yoke in the trans. The splines start about 1" in.

Nope, just a bad picture on my behalf. The angles really arent off that much at all and damn near parralel with each other.

Just by chance when they were installing the u-joint maybe a couple needle bearings got buggered up? Just thinking out loud.

Well, I would ponder that as well, except I had the driveshaft balanced and u joints put on a month after the vibration began immediately after the gear change. The ujoints and rebalance actually took 80% of the vibration away to its current level.

If the DS is indeed a bit short and any minor vibes now exaggerated by the recent gear swap, is there any major risk of dropping the DS in the meantime while sourcing a new slightly longer 3 inch piece?

The splines in the tail of the trans are all in good shape still and 3.5 inches of the 4.5 inches of total slip yoke in the trans is in the splines. Slip yoke in total is 6 inches long.

Assuming the circlips are all installed on the cups, my money is still on the shaft has some known but unquantified runout that has reared its ugly head now that driveshaft speed has gone north of 3000 rpm at cruise.

Also the yoke looks a little worn where it rides in the bushing which likely means the bushing has some excessive wear also. If you grab the yoke and shake it while it's in the trans, you should have to pay attention to feel if there is any slop as in less than .010".

If it has much more than that you should look at a new yoke and tailshaft bushing. If it seems excessive and you have another yoke that doesn't look as used as that one does, slip it in and see if it's tighter.

I went thru this with a Ford top loader after I went from a 2.75 to a 3.50. I changed the tail bushing which helped but my yoke looked like yours and when I swapped it out it drove like a new one after.

Kevin

One other thing to consider is what RPM the drive shaft was balanced at.

You need a high speed balance.

and most shops that do general driveshaft work for everything, [cars,trucks,big rigs] do NOT do high speed balancing. it seems that even though it costs a few dollars more, one might find a necessity to use a race type driveshaft company to insure the shaft is not only high speed balanced, but also to insure it has almost zero runout.

eyeballing your driveline angles is not valid

Measure them

come back when you have numbers

First, get a driveshaft that's 3/4-1" longer.

Second, you said less of a vibration and more of a hum... any chance that you're getting exhaust resonance with the higher engine RPM while cruising?

Yes, high speed balance. Totally forgot that aspect. Might want to consider that. Isn't high speed balancing based on high rpm harmonics rather than sustained speed?

Yes, I plan to check the actual angle…once I source an angle finding tool this weekend.

No, definitely not an exhaust drone.