Driveline Vibration

Thanks for the warm welcome......The vibration comes on strong between 35 and 40mph, will not go away until I slow down below 30. Vibration is driveline and or transmission. U joints are fine, weights are in place, pinion nut is tight, stock driveshaft has about 1.25 inch of clearance before yoke bottoms out into transmission. 4deg. of pinion angle in the right direction.
Tailshaft weight is not installed. Drive shaft looks straight when run up on jack stands. No leakage around tailshaft seal that would indicate a bad bearing.
What else could it be? Should I have the drive shaft balanced anyway?
Thanks for your help, Jack