a quick way to determine gear ratio:
-put truck in neutral
-jack up one rear tire
-rotate that tire around exactly twice while counting the times that the driveshaft rotates
-if the driveshaft rotates around 3 and a half times, you've got 3.55s. If it spins around just over 4 times, 4.10s
-if the truck has a posi or locker, then you need to jack up both rear tires and spin them around just once