* Front Differential: This is located in the front of the vehicle, near the front axle. The fill plug will be a bolt, often with a slightly different size or shape than the drain plug. Locate both, and you'll typically need a socket wrench. The fill plug is often higher than the drain plug.
* Rear Differential: This is located at the rear of the vehicle, near the rear axle. Similar to the front, it has separate fill and drain plugs.
* Transfer Case (if applicable): Your 4x4 Expedition also has a transfer case. The location and fill type will vary based on the specific transfer case installed in your vehicle (some use gear oil, others use ATF). Consult your owner's manual for the correct fluid type and location of the fill plug.
Crucially: Consult your owner's manual. It will have diagrams showing the exact locations of the fill and drain plugs for all three differentials, and will specify the correct type and amount of gear oil to use. Using the wrong type of fluid can severely damage your vehicle's drivetrain.
Before attempting this: Make sure the vehicle is on a level surface and the engine is cool. Use appropriate tools and safety measures. If you're uncomfortable with this task, it's best to have a qualified mechanic perform the service.
