I would jack up the rear end. Put the transmission in neutral. Mark the drive shaft and a rear tire with white marker or something. You then can rotate the driveshaft one revolution and have some else count how many times the tire went around or, it is easier to rotate a tire and count how many revolutions until the driveshaft has made one revolution. It should be roughly 3.5 (3.54) almost 4 revolutions (3.90) or a little more than 4 revolutions (4.10) etc. I have a Detroit locker in the rear and a Truetrac in the front. If I remember correctly it was not hard to lookup what size ring gear to purchase when I ordered the posi's. I own the very same truck and it came with 3.54 ratios. It is a little high for a 2.7. If you do have a 3.54 you can go lower to a 3.90 a lot cheaper than going even lower to a 4 something ratio. A 3.54 to a 4 something will require a new carrier in both ends and the cost really adds up.
