Oakie Bushing
Oh , I see ~ you didn't mention it is the bushing & seal in the _front_ end of the driveshaft , that's called an " Oakie Bushing " and some were made without a seal , glad you got it sorted out as that fixes a _lot_ of vibration problems and stops oil from draining out the tranny and overfilling the rear end .
Is it in yet ? if not , you'll need to freeze it so it'll drift right into place with a block of hardwood (from old pallets is cheapest) . some of the newer Oakie Bushings are poorly made , one fellow had to chuck his into a lathe and trim off like .020" to make it fit .