This thing eats u joints, there is two possible u joints for this thing and nether works. One is to big and the other one the caps are to small.

 

Sounds like somebody has swapped yokes or the whole rear end at one point in time. Take it to a driveline shop and get the correct parts. Better yet, get your shaft balanced and have a pro put QUALITY joints in it. Cheap joints wear out fast, especially if they're not put in right. Too tight and they cook, too loose and they wobble.