U joint failure
 

I have a 68' Polara, 383, 727 tf, and for some reason i have destroyed 2 u joints in the past few months. now if i was state side, i wouldn't care. but over here it is not easy to get my hands on one in a hurry. i plan on ordering a stock pile of the things, but it would be nice to know why it's happening. everything seemed tight, but both u joints lost 1 cap (not sure if it's the same one) while on the highway. I had the tranny rebuilt about a year ago. oh, and it's the rear u joint. any advice / directive would be appreciated.

Check to make sure that the ears on the yokes are round and not elongated. Also make sure that the driveshaft is straight and balanced. I had a friend that had a balanced shaft but it was bent just enough where she was going through a u-joint every 4 months.

X2. Also, if someone was a bit rough removing an old u-joint, the ears could be bent inward, or just on one side, pinching the u-joint, or putting it off-center.

Did you use New C clips ? Or are you getting the correct U joint. The U joint cap should not get lost !

Nothing against your abillities but how are you installing them?
As said they should be captuerd on all four points, by the c clips and should not be able to get out. Make sure you have the c clips inside the housings and behind the straps. If you loose a cap on the highway something is messed-up. I would think you would have noticed an out of balance drive shaft, as it would vibrate a lot.