"Another thing might be a worn tail shaft bushing but that is something I have not experienced."

Sometime ago, I had a badly worn tail shaft bushing on my 90 244. As I remember, there was a vibration but I can't remember if it worsened upon hard acceleration.

Here are two other symptoms to help identify a worn tail shaft bushing:

1. Grab the front of the drive shaft near where the it bolts to the tranny. Shake it back and forth vigorously. The driveshaft should have minimal or no movement. If it moves much the tail shaft bushing is toast.
2. If the bushing is too loose then the rear tranny seal should also leak ATF. Look for ATF splashed around in the area near the rear of the tranny.

I suggest checking this out before going to the trouble of removing the DS to check the u-joints...:)