Quote: 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.
, well 1st and second are smooth, in 3rd now with the car on stands i can feel a vibe at 35mph, i can feel this with my hand on the car, it comes and goes when i change rpms it will stop at certain speeds and rpms, then it will come back. At around 45 to 55 i dont feel it, the it starts to show up around 80. i dont see anything moving out of the ordinary. i measured the rear from side to side and the rear is 3/8" more to the passing side. if that matters or not., how bent would the rear housing have to be to cause this, or would it? i looked at the harmonic balancer and it doesnt even look like its moving, so i dont think theres an issue there, and the motor doesnt even look like its running thats how smooth it looks. if it was something thats not in or near the rear would it still cause only the rear ujoint to go bad. if it was something from the transmission tail shaft forward would it try to eat the front ujoint and not the rear. Im thinking its something in the back end cause it only eats the rear ujoint
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 
