Transmission. The axles and diff. have nothing to do with being able to rev. If you can go forward but not rev. then it's the trans. It might feel like it grabs, but the clutches might be slipping to much to make it move.
2003 Ranger FX4 level II. 3" BL, 33x12.50 GT Radial Mud Claws, Custom front and rear bumpers with winch hookups, custom flatbed, with hi-lift, and full size spare mount, 12,000 lb winch mounted under bed, Electric fan, and of course a flowmaster.