engine stalls with A/C on

Sounds like the idle speed is too low when the a/c compressor is on. The system should raise the idle speed a bit to compensate for the additional load of the compressor, but it doesn't appear to be. There can be many reasons for this and without running some tests and checking it out I would only by guessing. Best to have a shop that can properly diagnose this look at it. http://repairpal.com/gmc-repair-in-birmingham-alabama Good Luck

motor stalls when deforster is slected

it sounds like your throttle body is dirty and or built up with carbon i would try to clean that using a carb cleaner