Powertrain Problem - Vibration

I have an older 5-speed 325e. It was leaking gear oil so we went to replace the gasket on the back of the gearbox and found that the rubber "donut", for lack of a better word, that surrounds the driveshaft and bolts to the gearplate was disintegrating. We replaced it with a generic equivalent and refilled the gearbox with the best lube we could get, Red Line MTL.
The problem is that under heavy torque (accelerating hard in 1st or 2nd gear), there's now a very noticable vibration in the drivetrain. It feels like the driveshaft is gonna come right up through the floorboards.

Anybody with an idea of what could cause this? My mechanic said he reattached the driveshaft and matched the bolt pattern to what it was before, so it shouldn't be a balancing issue. Help!