Exactly what happens to the u joint? When you remove it does it show it was getting hot? Does it just come apart or break into pieces? Are the bearings disassembled and inspected? These joints are pretty tough and can take a lot of abuse along with a lot of miles but they can't run without proper lubrication. I've been messing with flatheads for 60+ years and I've seen u-joints getloose from many miles of wear and vibrating from being so worn out but I've never seen one break. Maybe I'm just lucky.