General Motors axle codes are found as a casting number on older GM axle housing or as part of the production codes on vehicles manufactured after the mid-1980s. GM axle casting numbers are typically etched into the axle housing tube and around the differential, depending on model.
Newer GM vehicles typically have a list of production codes in the glove box or driver's side door jamb. The production codes are alphanumeric and can be translated at sites such as SierraGear.com. The codes that pertain to axles include codes that begin with the letters "F," "G" and "H." "F" codes refer to the transaxle and front axle, while "G" and "H" codes to the rear axle.