Maybe the problem isn't the u-joints. When it grenaded the first time, did you drag it all over the highway? That usually happens. The yoke attached to the diff end of the driveshaft may be bent. Maybe the driveshaft itself is bent. Put her on a lathe with some dial indicators and see if it is out of round. While you are there, check both yokes.
I am willing to bet, assuming the driveline angles haven't changed, that something in there is bent up causing significant loss of life in the weakest part of the driveline: the u-joint.
Try going to a wrecker and finding a shaft that doesn't have grease nipples in the unjoint. No grease nipples = the old owner never had an excuse to touch it (it hasn't been dragged on the highway...) Take it home, rebuild it and install it.
Hope this helps. Give me some email either way.