Or at least count driveshaft rotations per wheel rotation. It won't be as accurate, but would be close, less trouble and cleaner. I'd only worry and explore more deeply if the front count didn't match the rear. It will give you a pretty good idea of what the ratio is. I doubt someone would go to the trouble of changing a 3:54 to a 3:71. I bet it would be more drastic than that.
The tag only tells you how it was built at the factory. Once it leaves... who knows? ESPECIALLY on a 30 some odd year old Jeep.
Paved roads are just for getting you to the starting point!