Have the same year truck and am having the same problem. I changed the u joint but I knew that wasn't the problem. But I have to take the drive shaft off and pull it apart before I know what it is for sure. Have you looked at the fluid in it?
universal joints don't leak. The needle bearings inside the caps go dry and fail...they are greased, not enough grease would come out of them to give you indication they are "leaking". Further investigation will tell you if your pinion or transmission is actually the culprit.