You can get the axle code off of the sticker in the drivers door jamb, then call the parts dept at your local dealer, read them off the code, and they can tell you the ratio. Another way is to jack up the back of the truck, put the trans in neutral, mark the driveshaft with a peice of chalk, mark a tire with a peice of chalk on the inside sidewall, then rotate the driveshaft until the tire makes one comeplete turn, count how many times the driveshaft turned to make one comeplete tire revolution, then round that to the closest available gear ratio. For example, 3.5 driveshaft turns would be a 3.55 ratio, 3.75 would be a 3.73, slightly over 4 turns would be a 4.10 or 4.11 etc. etc.
Another way is to remove the diff cover, and count how many teeth are on the ring gear and how many are on the pinion gear, divide the ring gear tooth count[big number] by the pinion gear tooth count[small number] and this will give you your gear ratio.