Mine was the front pinion seal. Ford replaced the seal under warranty. A year or so later it was leaking again. Independent shop found that yoke was pitted and corroded and was preventing the seal from working effectively. New seal and yoke, no leaks.
Changing the seal does take some skill as the torque on the nut that holds the yoke in place puts the pre-load on the differential gears. It needs to be done correctly.