Stalling

Sounds like there is a short with the cooling system. It is drawing to much juice to run the AC and pulling power away from running the engine.

I just cleaned the motor, so that could be the problem. The truck will run with the A/C on, but once I stop, the idle will drop and the engine will try and sometimes stall.

You might want to have the altanator checked to see what it is putting out. At a high level it will run the vehicle, but at an idle if power is going somewhere else, it will stall out. I would check the AC fuse, make sure it can handle the right amount of current, and check all wires to and from the cooling system. After that a certified GMAC dealer will hook it up to the computer. Is it still under warrenty? If not price it out.