If the cover is taken off the rear, the numbers should be stamped on the ring gear (on a 12 bolt anyway, not sure about a Dana 60 or an HO52).
They would be like 41:10, which would mean 4.10 gears.
Or you could jack the truck in the air, and count how many turns the driveshaft makes from one turn on the wheel. If it was about 4 exactly, you would have 4.10 gears, about 4.5 would be 4.56, ect.
Good luck
