I have my diff cover off and I seem to remember there is a way to count your teeth on your gears and do some sort of calculation to determine what gears you have.
just count the teeth on the ring and then on the pinion and divide the pinion into the ring. I think mine had 11 on the pinion and 41 on the ring= 3.727.
or jack the rearend up mark the drive shaft and the tire and spin the tire once and count the rotations of the drive shaft. if it turns just over 3 1/2 it 3:55's or if it turns 3 3/4 it 3:73's or if it turns just over 4 times it 4:10's and so on and so on