Possible causes: cooling fan(s) not working, water pump, bubble in the coolant system, blown headgasket.
Look in the coolant overflow tank. Do you see any black material? Take off the cap and sniff. Smell any exhaust-type smell? If so, that's probably a blown head gasket. Exhaust gasses are forced into the cooling system.
If the cooling fan isn't cycling on and off that usually means the fan or a relay is bad.
If you have large air bubbles in the system, you can elevate the nose of the truck a bit with ramps, a jack or a moderately steep driveway and then remove the radiator cap and run the engine until the cooling fans cycle. (Keep the cap off, but watch the temperature gauge and shut off the engine immediately if the temp starts to climb rapidly.) After that, shut off the engine and reinstall the radiator cap. You've now 'burped' the coolant system.
If none of that works or is applicable, I'd replace the water pump and possibly the radiator.