Did you check the smoothness of the u-joint while the driveshaft was out of the car? If the u-joint is stiff in one position, it needs to be replaced. If it is indeed stiff in the center in one or more axis, you should try putting a few washers between he carrier bearing mount and the body. This will cause the u-joint to flex at a slightly different area, and will sometimes correct a droning vibration.

Good luck,