Put it in 4wd high, and the transmission in neutral (if he has ADD, you will need the truck running when you shift it into 4wd and then shut it down before jacking it up).
jack up one side, and put a mark on the driveshaft with a marker. Spin the wheel in the air for TWO complete revolutions, and count the number of times the shaft spins. Just a little past 4, you have 4.10's, 4 and 1/3 on the shaft is a 4.30 gearset (not too likely), 4.5 turns, is a 4.56 gearset (aftermarket if he has it), and nearly 5 is a 4.88 gearset (could be with the auto).
Make sure you rotate twice. With one wheel in the air, the differential causes the ring to need twice as many spins to spin the pinion once as it does with both wheels spinning.
Cheers!