Ford 2003 Mustang Workshop Manual page 467

19. If the differential ring gear backface runout measurement, taken at the beginning of this
procedure, did not exceed the specification, proceed to Final disassembly in this procedure. If
the differential ring gear backface runout measurement, taken at the beginning of this
procedure, exceeded the specification, the cause may be a warped differential ring gear, a
damaged differential case, or loss of differential bearing preload. Proceed to Excessive
differential ring gear backface runout in this procedure to verify the cause of the excessive
runout.
Excessive differential ring gear backface runout
20. Place the differential assembly with the differential bearings (4221) and the differential bearing
cups (4222) in the differential housing.
21. Install a differential bearing shim of the thickness shown on the LH side of the differential case.
22. Install the LH differential bearing cap finger-tight.