After you get done checking other causes, consider this there is a right way to install a u joint. Most guys say, "Hell yes there is, you have to able to grease it!" I found out while farting around with lifted 4x4s with big tires and high torque, the grease fitting needs to be under compression. The grease fitting is a hole in the u joint, and if the driving yoke compresses that hole into the driven yoke they will suffer less failure. If the hole for the grease fitting is being pulled apart by the two yokes they bite the dust more often. Seems far fetched for your application, more like mud boggin with big tires. .02 A.G.