Wierd Vibration After New Trans Mount

[the_shootist]

I recently replaced worn out trans mount. When I did it, I jacked the transmission up to the point that it barely touched the body. After getting everything tightened up, I am having a vibration when taking off. I can feel the vibration in my driver's seat as if the transfer case is making contact with the body just below the seat. I measured the new trans mount and it seems to be the same height as the old one, and I can't figure out where the rub/vibration is coming from. Any advice on what to do or check would really be appreciated. It is really driving me nuts.

[Tripwire]

is it possible the bad mount was masking something? like a worn U joint and putting the tranny back into correct position now allows the sloppy joint to announce itself?

[the_shootist]

I should have mentioned that I put in new U-Joints at the same time. The U joints had actually been bad, which is what I think caused the tranny mount to get so worn.

[joe]

Did you mark the shaft with index marks when removed so you installed the driveshaft in phase?

[yossarian19]

The vibration is unlikely to be the mount. I think the U-joints are the culprit. See if the vibration comes back (probably less) at 30 mph or so, it's the classic u-joint issues speed.
I've had u-joints that were installed correctly and everything was pretty well fine but they were a bit stiff until they wore in a little bit and smoothed out. It's happened.
Could also be a problem like a dropped needle bearing I suppose or just a poor quality joint.
If you pulled the driveshaft, whichever one, and gave the yoke ends a wiggle you'd feel an uneven stiffness to it I betcha.

[the_shootist]

Thanks for everyone who chimed in. It turns out that it was a bad driver side engine mount. On acceleration, it would shift just enough to cause it to rub. I should have checked the engine mounts first off. I feel quite dumb for not doing so.

I think what happened was that when I did the lift, I had a vibration at low speeds. I drove it like this for a day or two and then added the shims. I think that vibration caused the already worn out u-joints to go, along with the already worn out trans and engine mounts. I am going to get the new mounts now and am certain it will fix the problem.