I'd put some fluid in it and go from there. Although it probably won't (go)depending on how long there was no fluid. LOL

 

My first thought was mismatched gear ratios, but then he said the ratios were the same. The project Z71 I bought binds like that (now that I got the 4wd working). It will slow down when you shift into 4wd, then you feel a jolt and hear a pretty harsh pop shortly after you notice it slowing itself down. It then feels like it frees up a bit, binds and slows again, pops again, and repeats that cycle until you say "oh crap, no wonder the last owner of this truck didn't try to get the 4wd working". OP, I was planning on getting a build sheet for my truck to see what the stock gear ratio was. I was then going to count the teeth on the ring and pinion gears on the rear axle and see if that matches the stock ratio. I'm 90 percent sure the rear axle on the truck I'm working on has been swapped.