there is a ring gear and a pinion gear inside the rear end.... it should be pretty visible..... the ratio of the two gears is your gear ratio.....
if you open the rear end, you can also read on the side of the ring gear (i believe), and the ratio should be imprinted on the side of the gear (if you hate counting teeth)...
if you are too lazy to do either of those, try this (this is what i did)... jack up the end of the truck, and turn one of the rear wheels a full turn. as you turn the rear wheel one complete turn, count the number of rotations of the drive shaft. BTW, use a piece of chalk to mark the tire and the driveshaft (it makes counting much easier)..... then do a little math....
here is how mine turned out:
1 turn of rear tire = 1-7/8 turn of driveshaft (or 1.875 turns)
2 wheels x 1-7/8 = 3.75 (which is approx 3.73)
thus, my rear gear ratio is 3.73.
if you think you have a 4.10 rear gear ratio, your driveshaft will turn just over two complete rotations for each single rotation of a rear tire (4.10 / 2 = 2.05 drive shaft turns)
hope this helps.
good luck!
jewels.