Is there a tag on the diff cover?
if not here is what I would do, it may take a helper to get accurate, mark the driveshaft, with the valve stem at the 6 or 12 o'clock position, you can rotate the wheel 1 revolution counting the driveshaft rotations, go slow, the shaft spins faster, that will get you the approximate ratio, you can get closer by spinning the wheel 10 times and counting the driveshaft revolutions.
If the tag is on the dif cover it can be decoded, I have a book but google will help too.