There are two ways to determine your differential. The first way is to use the axle numbers. This can come either in the form of an axle tag which is bolted onto the axle with the differential bolts or with an axle stamp pressed into the differential cover itself. Unfortunately, the ID tags aren’t always legible. If your  ID tag can’t be read you can identify your differential by the number of cover bolts, the distinctive shape of the cover, the number of ring gear bolts, and ring gear diameter. They can also be identified by whether the center section is an integral or dropout design. Dropout differentials are also referred to as 3rd members or pumpkins.