If it has not been taken off, there should be a tag on one of the bolts on your rear diff with some numbers on it. Look for things like 3.55, 3.73, 4.10. These are common ratios on pickups and Jeeps.
How To Find The Rear Axle Gear Ratio?
How do I find the diff ratio of a 9 inch it is off car and sitting on bricks someone told me you spin diff center and see how many times axles spin,something like that lol