Have an 2.8 V6 '87 LT auto that suffers from severe vibrations as it goes up the rev range. Vibrations drop off after auto box shifts, then pick up again as the revs rise before the next gear change. Doesn't do it when revved and not moving. So it has to be something to do with a part which moves at the same speed as the engine when the car is moving. Have had the engine and transmission checked twice to no avail. Mechanic reckons the drive shaft and rear diff are fine. Anyone out there got any ideas or experienced something similar? Am getting more and more frustrated by this because the vibrations are getting so bad it feels like the car is about to shake itself apart.

Thanks.

Maybe simple like you threw a wheel weight off. Can you feel it through the stering wheel?

Nope, steering is fine. The vibrations seem to come right up through the body of the car at the rear end, which is why I initially thought either the driveshaft or rear diff might be the problem.