being a 3500, you didn't mention if it was a dually. but i've seen up to a dana 80 w/ single rear wheels. best bet is to get the b.o.m. off a tag or the axle tube. b.o.m. stands for bill of material, and w/ it i can identify it, tell you what ratio it has, or if it has a posi. if does not have a tag, you can look to the right of the center section of the tube. stamped lightly on the tube will be the #, the numbers are also small. it usually is a 6 digit number followed by a space w/ another number single or two digits. example 606732 3 or 603598 12 sometimes grime can cover it up or rust use brake clean and a wire brush, or some 150 sandpaper.