I have a 99 ford expedition and the ac cools in the front not in the back. What can I do?

Pressure is 40 on low side 200 on huge side

You can take it to the Ford dealer and they will help you.

Take it to a shop for an inspection and testing.

http://www.ford-trucks.com/forums/1061026-what-is-the-most-common-issue-for-rear-a-c-not-working.html

Expedition AND Navigator both have AC/ Heater assemblies SEPARATE from the front system.

In the rear of the vehicle, Behind the "LEFT REAR" interior trim panel, you will find a the heating /AC unit. This contains the Blower motor with resistor, a heater core, & an evaporator.
Check for supply voltage there, as-well-as voltage to & through the resistor assembly, to the blower motor.

After 15 years, the connections in this DAMP enclosed system can develop corrosion. Corrosion acts as a resistor and can causes a high amp draw in the system, which can blow fuses, fusible links, or trip breakers.
Pull apart the connectors are make sure they are clean & then make sure they are tight.
There's also a resistor assembly that is used for low to high fan speed settings. Check that also

Then check all the fuses, fusible links &/or breakers

FUSES:
There are fuses & relays in the "Power Distribution Box" under the hood, mounted to the LEFT inner fender-well.
Here, there are miniature fuses, cartridge type fusible links, relays & circuit breakers.

In the passenger compartment, there's a fuse block behind the panel under the drivers side instrument
panel
There are also battery fuses NEXT TO the battery. Accessed by lifting the cover.

All fuses, breakers, fusible links & resistors can be checked with an ohm meter
( remove from power source before testing)