Shake, rattle and roll

if your seals are bad that can deffinatly make your driveshaft vibrate.

Check the U joints: They are a known problem with driveline vibrations. With the car on 4 jackstands, crawl under and see if you can shake the driveshaft or twist it by hand with the trans in gear or Park. Next try rotating one of the rear wheels and note any slack in the U joints while turning the wheel forward and backward. You may need a helper to turn the wheel while you watch the U joints for slack or movement. It may be necessary to block the wheel that you aren’t turning to keep it from rotating. Slack or movement of the U joints means you need new U joints, which are about $10-$12 each. Do the front and rear U joints at the same time.


U joint replacement:
1.) Remove the drive shaft, the rear bolts are 12 MM. You will need a high quality 12 point wrench for this: do not skimp or you risk rounding off the bolt heads. A helper to step on the brake to keep the drive shaft from turning will be helpful. Use your foot to apply pressure to the wrench to loosen the bolts. Note the yellow markings in the drive shaft and write down their alignment.
2.) Put a catch pan under the rear of the transmission to catch the fluid when you slide the yoke out of the transmission.
3.) Remove the snap rings from the bearing caps and it's now big hammer and socket time. Use a suitable big socket as a punch and another bigger one on the opposite side to act as a receiver. Pound away with the big hammer to drive the old U joint bearing cups out. The installation process is the reverse of the removal, except use a block of wood to hammer on rather than a socket. An air hammer is a great tool to have about this time, and it beats (excuse the pun) doing it by hand.
4.) Reinstall the drive shaft, slide the yoke in place. Align the rear yellow markings and install the bolts. A helper to step on the brake to keep the drive shaft from turning will be helpful. Use your foot to apply pressure to the wrench to tighten the bolts.
5.) Check the fluid level in the trans while you have the car up on jackstands. There is a pair of pipe plug filler ports on the side of the transmission. Remove the top plug and stick your little finger inside the trans: if has oil all over it, the trans is full of oil. If not, add Dextron/Mercon ATF (T5) or GM Synchromesh Black ( Tremec 3550, TKO, TKO 500 & 600) until it flows out the top filler port. Put a catch pan under the transmission to catch the fluid when you fill it to catch the overflow. A 4 ft length of ½” garden hose and a funnel is the tool for filling the trans with oil. Insert one end of the hose in the trans filler port and the funnel on the other end. Raise the funnel end up to level with the top of the front fender and pour away until you see some overflow. Put the pipe plug back in and tighten securely.

Get car down off the jackstands and go for a test drive. If the U joints were the problem, your vibration & noise will be gone. If not, it wasn't an expensive fix and you learned something in the process...