Why is my truck overheating

Sounds like the radiator cooling fans are not working properly. When you turn the air conditioning on the cooling fans are turned on as well. That's why the engine stops overheating with the air conditioning on. The problem is probably the coolant temperature sensor. It may have lost its calibration and is telling the computer the wrong engine temperature. Try this, with the engine cold, unplug the coolant temperature sensor and start the engine. The check engine light will come on, ignore it. The radiator cooling fans should come on immediately at high speed. If they do, replace the coolant temperature sensor with an AC Delco sensor. GMs don't like aftermarket coolant temperature sensors for some reason. Also, make sure the cooling system is full. Hope that helps! Jim