HouseCallAuto 4/2/2010There is a TSB 06-4-4 that talks about the rear differential chatter. I suggest that you ask Ford for a printout of it. And the plug is back there, it has to be.
skjdave 4/3/2010The fill plug is located on the front or leading side of the differential housing 2/3 up from bottom. It's hard to see but you can feel for it.