Thanks for the discussion here. You saved me $80. The guys at the oil change place tried to pull this one on me this morning. Showed me some plug covered with gunk and small metal shavings. It's the second time they've tried to get me to have the service performed. Luckily, I pulled out my owner's manual, and told the guy I'd see what Ford recommends. Hmmmmm, didn't see a thing in there about rear differential fluid. I told him I'd do some research and have it done next time if I thought I needed it.
Time for me to find a new place to have my oil changes done!
Thanks, Dawn in South Carolina
2000 Ford Explorer XLT