This months issue of Hot Rod (April 2017) has an article helping a guy with the same problem your having,only his was worse. Hot Rod Rescue is the article. He went through everything, even taking it to a 4x4 shop because he was being told his angles were off. It was a combination of a lot of things, including transmission dowel pins out of alignment. Right now your just speculating and throwing money at a problem hoping to find a cure. Pick it up and read, very enlightening.
My dad actually showed me this article yesterday and we were looking it over. Very enlightening. Pinion angle is taken care of and confirmed a non issue as of yesterday. Alignment all looked good.
I am ok with the money I had spent so far, since new back tires were needed and a more robust driveshaft was preferred.
