The code will be stamped on one of the axle tube, couls be either side, on the bottom adjacent to the carrier housing. It'll either have 3.07, 3.73, or 4.11. If it's a 3.73, it may not be too bad depending on what tire size that's going to be run. If he's not totally committed to the 4 speed, perhaps a T5 might be a better option. That'll get an overdrive gear for highway use.
