Vibration

OK, my friends stock 93 XJ has got a vibration that starts around 45MPH and continues when accelarating or holding steady, but quits when you let off the gas. On the gas= vibe, off the gas= no vibe. The u-joints appear tight. Just serviced rear diff too, made no difference. The T/C was way low on fluid though (it's an NV242), hopefully something in there isn't screwy. Thoughts? Maybe a weight on the driveshaft came off? The only thing that has happened prior to this vibe is she had the tranny serviced (don't know why they didn't check the T/C and see it was way low). They looked at it too when she took it back with the vibe and they don't have any ideas either. ???

pull the driveshaft and put it in 4wd.
Drive it at speed as a fwd. It will handle funny.
If the vibes go away it's likely something in the driveshaft.
If they don't, you're chasing the wrong part.

It's got a slip yoke, gonna be a lot of lost fluid if I pull the driveshaft.

I use a ruber plug to plug mine if I have to pull the rear shaft.
Do you have anything similar that will work?

Have you checked the motor mounts? They tend to wear out causing the symptoms you're describing.
Mark