Sounds like the TC is good and probably the tranny also, check the rear differential. Crawl underneath and check the trasfer case for cracks. Also look at the rear differential, two ways to do it. One, take the cover off and look at it, but then you will have to refill it, another way is to take the rear drive shaft off and then put the trasfer case into 4wd and see if the truck moves. Also, with the drive shaft off and the back tires off the ground, try to spin them by hand.