Ford rear ends, or differentials, were manufactured by either the Dana Corporation or Ford. Differentials take the power from the rear axle and transfer it to the rear wheels by way of special gears. Identifying a differential hinges the ability to distinguish the two primary rear-end types---the 8- or 9-inch rear end---denoting ring-gear size. The differential installed depends on the intended application; standard passenger or high-performance trucks tend to have a wider range of rear-end variety than passenger cars. The identification process begins with locating the factory rear-end tag and then using visual methods.
Locate the axle ID tag on the rear-end housing---usually a sheet-metal tag bolted to the housing, although some vehicles had a paper tag, which may be missing. The metal tag has four different sets of numbers, each grouped in the four corners of the tag. The most important is the model, or service code, found in the upper-left corner. Combine all of the codes, to identify ring-gear size, gear ratio, date and location of manufacture.
Decode the service number by matching it to an axle service code chart, similar to those found on WoodyG.com and Drivetrain. The code chart tells the gear ratio and ring-gear size, denoting an 8- or 9-inch rear end.
Locate the VIN tag on the rear face of the driver-side door---the primary identification tag for the vehicle. On it, look forums box labelled "axle," containing the axle code, which requires consulting a code listing similar to that found on Drivetrain. Verify both the axle tag and door-tag codes to determine whether the vehicle still has its original rear end.
Count the bolts on the rear-end housing, and visually identify the rear-end gasket shape; compare the gasket shape with the chart found on the Drivetrain site (see References section). Ford 7.5s and 8.8s have 10 bolts, the 10.25 has 12 and all Dana units have 10 bolts, but each has a different gasket shape. The chart also provides width dimensions for measurement verification.
Attach a deep socket with an extension to the lower rear-end bolts. According to Ridgecrest, if the socket goes straight on without hitting the case, your vehicle has an 8-inch rear end; if the socket doesn't fit the bolt on the other side of the centre, your vehicle has a 9-inch rear end.
The paper tag contains the same information as the metal tag. If difficulties arise in model number identification, contact a Ford parts department with the exact information found on the tag to identify the rear end.