hello fellow brickboard community.

Lately my 1989, automatic 240 wagon has developed a vibration in the drivetrain / rear axle

It vibrates most noticeably in the automatic shift leaver anywhere from 35 mph - 50 mph after that it calms down to almost imperceptible. it vibrates coasting as well as accelerating.

you can also feel the vibration in the seat of your pants and if the passenger seat is empty the back of the passenger seat gets a good rumble going on.

details:
- i've eliminated the possibility that it is a warped virgo rim, by having them rebalanced and also swapping in my winter steel rims and the problem is still there.

- i've jacked up the car on jack stands and had a friend on the accelerator and cannot replicate the problem with the car in the air, in fact the drive shaft spins so smoothly through all the gears in all rpms and speeds that it's hard to believe that such a smooth drive shaft is responsible for such a bad vibration when the wheels are on the pavement under load. (the rear axel was under load with jackstands on the axle to replicate the same angle as when the car is on the highway)


any suggestions ? i've read the posts on here so far about rear u-joint failing first, i'm hesitant to disconnect that and turn it in my hands, cus i'm not convinced it's my u-joints, but i suppose anything is possible. also my tranny mount seems within spec and is not dead or too compressed. rear tabs and front truck bushings, replaced 4 years ago, torque rod bushings, shock absorbers about 4 years old too.

any help would be appreciated.

Thanks,
Jonny