What gears did the axle originally come with ? If it was like 3.08's, then originally you had a series 3 carrier. If it was like 3.73+, then it was orignally a series 4 carrier. If are installing aftermarket 4.10 gears, they came in thick or thin versions to go on either a series 3 or 4 carrier. Thick 4.10 gears need a series 3 and GM 4.10s and thin 4.10's need a series 4.
I have a series 4 carrier for sale.
Blake