To prevent clunking,lol
Naw,Sometimes there is a bit of vibration coming from the front of the driveshaft due to the U-joint angle speeding up and slowing down twice on every revolution. And the faster you go the more vibration there might be. The weight stabilizes the trans back end and damps the vibration, while the driveshaft is whipping around.
If the rear mount is good how about the driver's side engine mount? Maybe the engine is popping up and falling down on the shift.
