The GM 10.5" axle, commonly known as the GM "14 Bolt", is an extremely popular rear axle due to its strength, availability, and healthy aftermarket. They feature a massive 10.5" diameter ring gear and pinion with a 3rd bearing at its nose that prevents gear deflection under severe loads. Pair that with extra large 1.59" diameter 30 spline axle shafts and you have an axle that requires very few modifications to handle extreme applications.
If your axle still has the original factory gears in it, then 3.42 to 4.11 gears will be on the lower carrier and 4.56 to 5.38 gears will be on the higher carrier. Otherwise, use the table below to determine your differential carrier:
Learn more about differential carriers: Crawlpedia article on thick gears and carrier breaks.
If your axle still has the original gears installed and you do not know your current axle ratio, you can find it using your vehicles RPO code which is typically located in the glovebox. Find the code that starts with the letter G (or H) and look up your gear ratio in the table below:
View a complete list of RPO axle codes: Crawlpedia GM RPO Axle Code Guide.
Gear ratio, ring gear rotation, and thick gear designations can be found marked on most gears.
Calculate your gear ratio by counting gear teeth: Crawlpedia Ring Rear Ratio Calculator.