Fan Clutch

Does it have an electric fan? I'm not too familiar with trucks? If the fan is not kicking on, it'll overheat. Does it happen only in traffic? Or even on the highway? How do you know the system is overpressurized? Remember, overheating is one of the worst thing that can happen to a motor.

Usually when the clutches go bad, they freeze up and the fan turns the speed of the egnine. Load roar when the engine turns up speed. U should be able to see if the fan is turning by running the engine and pulsing the throttle. You can tell if the fan is increasing speed.

It almost sounds like you have an obstruction blocking flow in your system. Nobody has ever put any stop leak in the radiator have they? I know that stuff has been known to clogged the orifices in the head gasket. This in turn increases cylinder head temp, and eventually will create more heat than the system can take.

How does your radiator fluid look? Is it brown with floaty thingys in it? If this is the original engine, it may be time for a top end job. Give you a good chance to remove the heads and have access to all of the cooling system.

Im not saying tear your engine apart, but atleast do a compression check and make sure you dont have something going on with the heads.
Good luck, and let us know what fixes the overheating problem.