or, if you don't wanna take off the diff cover, you can use this method which will get you close enough to figure it out...
1. put a mark on the pinion yoke, and one on the axle housing (use something visible from a distance)
2. now put some sort of mark on the tire and something on the body (if you think you can remember the exact point, do that, if not, use some tape or something...)
3. jack the rear end of the truck up and secure it on jackstands, chock front wheels.
4. put truck in neutral and, while looking at the pinion (you should be able to see it through the wheel well if you get at the right angle...if not, then change your marks to the inside of the tire and an inside reference point on the truck) slowly rotate the tire while looking at the driveshaft...pay attention to your tire as well, when it rotates around once, you should have counted a few rotations of your driveshaft...here's an example...tire was rotated one full revolution, driveshaft spun around ALMOST 4 times...you probably have 3.73 gears. get where i'm going with this?
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