Do you hear the fan "roar" when the engine is hot?
The cooling fan will engage when the engine gets hot enough and if you rev the engine you will hear the distinctive roar... If the fan clutch is not working you can also tell if you turn the engine off when it is hot and spin the fan and see how far the blade turns after you turn it loose...If the fan seems to freewheel when it is spinning and the truck is hot your fan clutch is done...
The fan clutch is designed to fully engage under heat load and the a/c is adding to that heat load and it is not pulling enough air through the cores to adequately remove heat from them...With the a/c off the fan may be pulling enough air to cool the radiator though...
I wouldn't overheat the engine too many times...Heads and head gaskets like to go boom after awhile...
