Take a look down in the radiator if you can. Look at the oval tube openings. Are they clean, or are they gunked up? With the truck running and the thermostat open (check the top radiator hose to make sure its really hot), rev the engine. Does the level of the water in the radiator change? If not, there is a good chance that its a water pump issue. Another way to help diagnose a water pump issue is by buying a cheap laser/infrared thermometer. Check the coolant at the neck where the bottom radiator hose comes into the engine, check the thermostat neck, then check both sides of the radiator. If you have more than about a 30-40 degree difference in temperature, it might be a bad water pump.