Analysis of Leakage
WARNING: This page is about a different car, the 2013 Ford Explorer. However, it is still accessible from the selected car via links, so may be relevant.
Clean up the leaking area enough to identify the exact source.
A plugged RDU housing vent can cause excessive pinion seal lip wear due to internal pressure buildup.
Verify the RDU lubricant level is even with the bottom of the fill hole.