4WD System could have been bound up which didn't allow it to disengage the first time. Don't think the auto locking hubs on the Ford have changed much. If not they are basically the same as on my '92 and on more than one occasion I had to back up two or three times to get them to disengage.
My question is why you had it engaged on a road that was dry enough and fast enough to be washboarded? Sounds like the road could easily have been done in 2wd. A dry dirt road will not provide enough slippage with a heavily loaded vehicle to allow for the differential in tire rotation during curves.