On my '02 XL7, I have two issues. 

1. When I disconnected the air line from the front diff (at bottom of diff at toward back of car), gear oil drips out.  Is that normal or is the diff bad?

2. I cannot hear the air pump run while in 4x4 mode, motor running, car in drive, 4x4 light on.  When I disconnected the harness plug to the pump, I found three pins.  ABC.  A. pos+12v, B. ground, C. ground.   When I put car back in 2wd mode, I get the same results at the pins.  However, when I ran 12 volts directly to pins A and C only, the pump runs fine and air comes out.  If there is a relay for the pump that may be faulty, I have no idea where it might be.  The owner's manual is not helpful at all. 

Please help if you have any light to shed on these issues.  Thank you so much.

The easy thing to do would be to bypass the factory wiring and run the power through a switch on the dash. Not only would this fix your problem but it would effectively give you a 2lo option when the T case is in low range. Dave