1997 Ford Expedition stuck on 4WD

First, try to avoid driving in 4wd on dry surfaces. Next, what you need to do is check to see if the 4wd actuator is getting power when you switch from 2 to 4wd. THe actuator is located on the transfer case. If it is getting power but not shifting for you, chances are the actuator is bad. If there is no power, check the wiring for brakes or corrosion and work your way back to the switch itself.

Let me know what you find.
Joe