I just swapped a 3:55 sure grip into my 8 3/4 rear (had a 2.94 sure grip before) and now I'm getting a noticeable driveline vibration when coasting downhill in gear (under load/uphill it's less noticeable), in neutral it stops. I tried flipping the u-joint/driveshaft 180deg but same thing. It seemed like the pinion/nose was pointed up about 7 or 8 degrees after the swap, but that seemed different to me than when I was originally taking the chunk out - I remember it looking basically flat. Could I have tweaked the angle of the housing by jacking the car up by the axle when I had the pumpkin out, like maybe it slipped in the u-bolts or something? Could this cause the vibration? I read somewhere if the trans is pointing down at the rear a certain # of degrees, then the pinion should point up the same number of degrees. Or they should both point down the same # of degrees... or something like that - I forget which. Is this something I can adjust by loosening the U-bolts and rotating the axle a little? Car has six-leaf springs but pretty soft shocks, at the moment. I've never had to adjust this before when I swapped pumpkins but maybe I got lucky? Or could it be the universal joint?

Your axle should not be able to rotate, there is a perch welded to the axle housing on each side that prevents that.

I suppose the Ubolts could be loose though.

When setting pinion angles you want the output shaft of the trans an the pinion of the rear to be parallel under load. So typically your pinion will be nose down a couple degrees at rest.

yes, recheck the ujoint/pinion flange fitment. EDIT in neutral & moving it is OK? Nothing else was altered/changed?

The only other factor is a slow drip leak from the diff main seal. I topped up the gear oil before a test run (after flipping the driveshaft 180deg, which actually helped a bit, but not completely). Afterward I noticed fluid leaking from the pinion, which is weird since the whole pumpkin is brand new (actually, bought a few years ago from a reputable source; it has been well-stored in a sealed bucket).

Could the driveshaft vibration (or whatever it is) be damaging the pinion seal? Or maybe I overfilled it and it's just spraying out from a seal leak onto the pinion?

When I originally filled it I put in a tube of Trans-X additive and then it took about 2 quarts of gear oil before it started coming back out the filler hole. This is a 741 case with cone type suregrip and the small pinion yoke.

Also one of the u-joint caps fell off while removing the driveshaft - I straightened out the pin bearings and added a dab of grease, pushed it back on. Is there more to it?

All of the u-joint "C" clips in place?

And by checking the u-joint-pinion fitment, what exactly do I check for? The straps and bolts are pretty old, would new ones make a difference?

^^ I'll check the C-clips - are they supposed to keep the cups in place?

So, the "C" clips are there correct? They better be, as they are what center the joint. And yes, the straps as well as the flange itself can be worn enough to allow joint caps to move around even when bolts are tight.

Might want to have the DS checked as it is spinning a lot faster at a given speed with the 3.55's. One thing to eliminate. But the fact that you say it changes with load kind of indicates something else

Wondering if you have a 7260 U-joint, and the companion flange (yoke) on the new pumpkin was is a 7290? That'd sure do it!

Also, the "straightening" of the needle bearing in one of the caps sounds ominous.

Rick

^^ If the u-joint is bigger on the new pinion, wouldn't the bolt holes be farther apart? e.g. the old straps wouldn't fit?

Are you sure all the pins were back in? Or not got pushed into the bottom of the cap and cap not square now?

That's not what RE said.
A 7260 joint (smaller) in a 7290 yoke (larger) will cause problems.
Not only are the 7290 strap bolt holes further apart and will allow the 7260 joint to go in, the 7260 cap diameter is also smaller so the 7290 straps will never fully tighten up.
A 7260 joint in a 7290 yoke will bolt up but give a loose fit.
A 7290 joint in a 7260 yoke would never bolt up due to the caps not seating in the yoke from the larger cap diameter, and the straps being too short.

FYI. cap dia. 7260-1.078". 7290-1.125"

You say that it is a new chunk, Used but new to you? I have seen the yoke spread so the U-joint was not seated centered in the yoke causing a vibration. You should check that the cups fit tight in the yoke.

Make sure by measuring the yoke and the caps you dont have a miss match of ujoint size. With a 741 you can replace the pinion seal without taking the pig out of the housing. The pinion preload is done with shims. Take off the pinion nut, remove the yoke, r&r seal. Then reinstall yoke pinion nut using blue Loctite. Make sure you didnt lose any needle bearing in the cups. The c clips locate the ujoint between the yoke ears.

Also you driveshaft speed increased by 18% which is significant. Here's an interesting video as to pinion angle
https://youtu.be/gmV4qwLfOMY

Fantastic info - thank you... Raining today but might clear up tomorrow so I can get back under there and check it out.