Transfer Case
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 1990 Ford Ranger, 1990 Ford Pickup, 1990 Ford F-Super Duty, 1990 Ford Bronco II, and 1990 Ford Bronco. However, it is still accessible from the selected car via links, so may be relevant.
Remove filler plug. Transfer case is full if fluid drains out or is level with opening. Add lubricant as needed.