The Stovebolt Site Forums The Shop Area Driveline vibration

It is very common for the U'joint attaching bolts to come loose giving you a vibration.
There is also a very common problem I see. The front of the torque tube where the bell slides becomes worn, as does the inside of the part of the bell that slides on the torque tube. In this case the vibration smooths out on hard acceleration as the torque tube lifts up in the front putting the U'joint back in line.