This is a common problem on Ford Expeditions, Explorers and Windstars of this vintage. It is one of 2 things, a stripped actuator motor gear or a broken blend door. The actuator motor replacement is not too hard and the part costs about 70 bucks at the dealer.
If you fully open the glove box, you can see the actuator motor to the left of the opening. It is white and held on with three 8mm screws. To get a full view of the plenum box and actuator motor, it is best to remove the center console control panel. The hard part is removing the radio. There are two holes on each side of the radio module. These holes accept a special tool that will disengage the clip mechanism and allow the radio to be removed. The tool is just a “U” shaped wire that is inserted into the two holes and pushed together to disengage the clip. You need two U’s. The tool is available from any parts store. You can always go down to any car radio installation place and they can pull it quickly and probably won’t charge anything if they think there is a chance of selling you something. With the radio out, there are two screws at the top of the center bezel and two at the bottom that have to be removed. On some models, the console between the seats may have to be unbolted and pulled back. The panel will pull out and can be completely removed by disconnecting the different wiring connectors to the HVAC control module and the different lights and functions. With the center console panel removed, you have fairly easy access to the actuator motor. Remove the screws holding the actuator motor module in place and pull the motor back to remove it. Watch for plastic bits that will indicate a broken axle on the belender door. With the motor removed, you can use a large screw driver to turn the axle, checking for smooth movement and a hard stop with about 90 degrees of movement between the full open and full closed positions of the door. If there is a “spongy” feel at either extent of movement, this is a good indication that the door is in the process of breaking and needs to be replaced.