It is time to start isolating the cause or causes. Jack the car up(both ends) and put it on jack stands again under the suspension so all the weight is on the jackstands like it is on the track. Run it in gear first to make sure where the vibrations start and stop, RPM or MPHs . Shut the motor off and remove the rear tires, put all of the lug nuts back on tght enough to hold the brake drums on and test it agian, if the vibration is still there remove the break drums next and test agian. If thats not it do the axles next and retest again. The last thing to test is removing the driveshaft and that will allow some fluid to leak out. If none of those cure it then it is in the remaining parts, converter, trans, engine and so on. Test, test and test some more There is no magic in these parts, some problems are harder to diaignose than others Please let us know how it goes and what you find. BTW, are you lubing the new U joints with a good Hi temp wheel bearing grease before installing them If not do that next time and every time. U joints come with a rust preventive grese like cosmoleium (SP?) in them that is not intended to be ran on the road. I learn that the hard way One other thing to check is the pinion yoke and U joint alignment to the center of the pinion shaft, I had one pinion yoke that would not center the U joint which made the drive shaft wobble a very small amount.