Hi, as far as, I know their is not compressor for the 4X4 in the XL7, look like your front wheel lock system is malfunctioning.
This is my guess with the limited info you have given, looks like an electric circuit breaks when you break, and the from wheel does not work, If you do not have a electrically locking system, than you have to mannually lock the front wheels.
The mannual system works by turning the centre of the wheel lock system, these are there for fuel economy.
1. At the transfer case switch, check to see if there is 12 volts to the switch from the computer with the switch unplugged. Check that the voltage goes to 0 when the transfer case lever is moved to 4 wheel drive.
2. At the 4 wheel drive control unit, check to see if there is 12 volts at pin 4 Black/Orange wire.
3. Check for air leaks between the Air pump assembly and the differential.
4. Apply 5-7 PSI air pressure to the differential actuator. The actuator should lock the axles to the differential. If not, there is an internal problem.