Ok so I use my '66 as a daily driver, and recently I noticed an oscillating high pitch sound whenever the car was moving in reverse. So this past Saturday I figured it would a be a good idea to replace the u-joints. After running to the parts store I jacked the rear of the car up and placed it on jack stands (with tire chocks for the front tires). I then unbolted the drive line and pulled it out. With the drive line on the work bench I proceded to remove and inspect the u-joints. I found that one of the lobes was almost completely rusted solid!!! Then I noticed that the seals on the u-joint were cork. So the only thing I can think of is that they were the original u-joints the car shipped with or were replaced shortly after the car was purchased.
So I replaced both u-joint and no more squeaking when in reverse. I was just wondering if I am right about the age of the prevoius u-joints. Sorry i did not get pictures as i wanted to finish fixing the car and then cleaning up.