OK, lets try the simple fix first. As mentioned previously, You probably have air in the system. This is very common when the cooling system is drained to replace the thermostat. Try removing the radiator cap and start the truck. Let it run until the temp comes up and the thermostat opens. you can tell it is open by watching the coolant inside the radiator, you should see it start to flow around inside the radiator. Shut it off and let it sit with the cap off for an hour or so. replace cap and drive it. If this doesn't work, buy a new truck.