Blacksilo,
If you don't have any luck with the door sticker or the tag on the rear or front differential, you could always count the gears!
Pull off the rear differential cover and drain the oil into a pan.
Count the # of teeth on the ring gear.
Count the # of teeth on the pinion gear.
Divide the pinion gear count into the ring gear count, this will give you the ratio.
Scoop the remainder of the gear oil out with your hand checking for metal debris.
Check the ring gear wear pattern and pinion shaft free play by turning the driveshaft back and forth without turning the ring gear.
If all looks good, clean up the cover and mating surface, apply gear oil silicone sealer to the cover and reinstall.
Fill differential with 90 weight gear oil to bottom of filler plug and replace plug.
This is a good learning experience and it's good to know all is well back there the next time you're hammering thru the boonies!
Plus it probably was overdue for a change!
Good luck!