WARNING: This page is about a different car, the 2004 Ford Pickup and 2004 Ford Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.
Remove the differential housing cover.
Using a suitable dial indicator and the special tool, measure and record the differential ring gear backlash in 4 opposing points on the ring gear.
Fig 1: Measuring And Record The Differential Ring Gear Backlash