78D200

depends on if there is a tow package and what rear axle you have. There should be a tag on one of the bolts that holds the diff cover on. Another way to tell is to count the ring gear teeth and divide it but how many pinion teeth there are. Another way is to jack up the rear end, turn and count how many turns the yoke turns by the time the one tire that is up in the air turns one full turn. Another way is to rear the numbers stamped on the ring gear.

I'd be willing to guess 3.23 or 3.55 gears though (somewhere around there).