Try this:
"Jack up one tire if you have an open diff, or both tires
if you have a working posi or locking differential. Rotate the
tire one full revolution for posis and lockers and 2 full
revolutions for open diffs. Carefully count the number of full
revolutions the driveshaft makes. This is your gear ratio. In
other words, if the drive shaft turns 3 & 3/4 turns, you
probably have a 3.73 gear ratio. Turning the tire for twice the
number of full revolutions and dividing the drive shaft
revolutions by two will give you a more accurate
reading." (quoted from West Coast Differentials)
