Open the rear door hatch. there are 2 or 3 Philip screws on top of the brake light housing. remove them , pull housing from the frame of the truck. change bad light bulb, reinstall everthing you just took out and your done.
Check your owners manual. If you don't have one copy & paste this link. http://www.genuineservice.com/genuineservice/en/gloveboxresults
Try changing the bulb that doesn't have the low beam. Check at your local auto parts store for the correct bulb.
Probably the rear bulb.
On a 2003 Ford Expedition : There is an ABS warning light to the left and slightly lower than the " E " on the fuel gauge , which you should be able to see when the ignition key is turned to the RUN position , just before starting the vehicle ( to prove that the bulb still works ) so it looks like a 2003 Ford Expedition does have an Anti-lock Brake System
If I recall correctly. It should be a peanut light bulb. And if it is a peanut bulb they just pull straight out.
change the light bulb,
May sure the housing is good and not Corroded. If it is Corroded you may need to replace it.
Have you checked the bulb , it has 2 filaments in it , see if one of the wires is broken
Very carefully
bulbs or bulb sockets
may be the connector that the bulb plugs into