Ford 2003 Mustang Workshop Manual page 389

Tighten the special tool to specification.
9. NOTE: Offset the special tool to obtain an accurate reading.
Rotate the special tool several half-turns to make sure the differential pinion bearings seat
correctly and position the special tool as shown.
10. Install the special tool.
1. Position the special tool.
2. Install the bearing caps.
3. Install the four bolts.
11. NOTE: Use only flat, clean drive pinion bearing adjustment shims (4663).
NOTE: Selection of too thick a shim results in a deep tooth contact at final assembly. Do not
attempt to force the shim between the special tools. A slight drag indicates correct shim
selection.
Use a drive pinion bearing adjustment shim as a gauge for shim selection.
After determining the correct shim thickness, remove the special tools.