I got in a road race with a chevy pickup about a year ago, he was in a 350 / 5 speed truck and I was in a 305 / 5 speed truck and I was in the process of passing him at around 115 mph when sparks and all kinds of junk came out the back of his truck. He had a buddy just put U-Joints in and he wasn't able to get one of the clips in. The fellow said that the truck had some vibration to it after the U-Joints were put in. So what I am saying is the the harmonics fatiqued the shaft untill it broke into and I think when Allen gets the vibration problem solved he will quite breaking shafts, I know it would be a hastle but he might have to try another transmission and if that doesn't fix it he might borrow someone elses rear end just to see if the problem can be pinpointed.