Transfer Case
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 1989 Ford Ranger, 1989 Ford Pickup, 1989 Ford F-Super Duty, 1989 Ford Econoline, and 1989 Ford Cab & Chassis. 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.