I was experiencing something similar when my machine started to leak and the fluid in the radiator core got low. She would overheat slightly at idle (225 degrees) though would remain cool at highway or side-street rolling speeds. Check your radiator fluid level when she is dead cold, like sitting for at least 4 hours or overnight. Do not rely on the overflow bottle at all in this case, but what you can see (or cannot see) when you look under the radiator cap. Top her off with a 50/50 mix of coolant to distilled water, then turn the truck over. Let it idle until hot and the thermostat opens. Refiill any fluid that is "lost" when air bubbles out of the system with the engine still running - top it off to the radiator filler neck, and pop the cap back on. Refill the overflow tank to the hot/full mark and let it run a bit longer so the system presurizes.
Inspect for leaks or smell of coolant, fluid dripping on the ground, fluid around the end tanks of the radiator, hoses and heater lines. Shut her off and let it sit, continuing your inspection at the points above. Check for greasy, humid spots around the defrost vent and the windshield glass just above it in the cab, and of course, look for anything leaking on the ground. Check your oil level too - elevated levels could indicate an internal leak that's dropping coolant into your crankcase. I just had this happen to me from a combination of heater core hoses that were shot in the engine compartment, and lower, leaking manifold gaskets.