I haven't had it looked at yet but like I said I'm almost positive that its the shift motor. For the price of a diagnostic at the dealer I can pretty much pay for the cost of a shift motor. Doesn't the expedition have 2wd mode as well? If you can't shift between 2wd and 4wd hi or low then it may be your shift motor as well.
07 Ford Explorer Sporttrac Limited V8 4x4
-Autostart remote starter, Tinted windows, and soon a western bull bar
98 Ford Ranger 3.Slow Stepside
-16" Explorer wheels w/ Dayton AT's, Venstshades, Sunshade, Bugdeflector, and Custom Audio System