Does the vibration happen when the car is not moving, but the engine is running? Then possibly the flexplate is improperly balanced.
If it happens when moving, then there might be something wrong with the driveshaft angle. Also check
The front yoke/slipjoint is proper outer diameter and good spline inside?
The output shaft is straight?
The tailshaft housing is not warped or cracked?
The driveshaft is not bent?
The driveshaft is properly balanced?
The u joint is properly installed and is not missing ay bearings?
The bearing (not the bushing) at the front end of the tailshaft housing is good and properly seated?