If you remove the access panel on the inside of the tailgate, you can check the condition of the window regulator and tracks. I had a simialr problem when I first got my 84 Bronco, and I think it just had alot of dust and grime and rust on the regulator. It took a whole can of WD-40, but I sprayed down both sides of the ends of the glass and the arms on the regulator. Worked the window up and down, and helped it along by hand with the aid of a friend. If the gear in the motor is stripped, you should still be able to help it along by pulling or pushing on the regulator. Just watch your fingers, and watch out for any critters living inside the tailgate.
