time to get a jack and jackstands out...
get the front end off the ground and safely on jack stands - both tires off the gound.
-leave transfer case in 2WD, but DO engage the front hubs. spin the front driveshaft by hand... Popping? grinding??? try it with the wheels turned left, and the wheels turned to the right.
if all seems well,
-get both rear tires off the ground and safely on jackstands.
-put it in 4WD (with front hubs unlocked)
-spin either driveshaft by hand... feel any popping or grinding in 4H?, anything in 4-LOW??
-if so, inspect both driveshafts, but start pointing fingers at the transfer case
if nothing is blatantly obvious by then, I would get some chalk, mark the top of both tires, mark the driveshaft, and start counting the number of driveshaft turns it takes to get the wheel to go all the way around in a complete revolution. do the front end, and then the back end. you want everything in 2wd/neutral and when doing the front, hubs locked. this could tell you if the gears are mismatched.
(make sure both tires are turning at the same time in the same direction though- if a brake is dragging more on one side than the other, this will get off course)
