Overheating

How high did the temperature get? Are you 100% sure that you got the thermostat in correctly? Also, it is not unheard of to get a dud of a thermostat from the parts shops. If it is installed backwards, that would definitely cause the truck to overheat, as it cannot open and allow coolant to flow and cool the engine.

Start the engine and let it warm for a few minutes, watching the temperature gauge. Once it is around operating temperature, shut it off. Carefully feel the upper and lower radiator hoses. If one is warm/hot and the other is not warm at all or is barely warm, then you have some sort of blockage in the cooling system. Most likely cause is a bad or improperly installed thermostat.

If you checked both hoses, and they weren't warm at all but the engine was overheating, then you have to be low on coolant. Seriously low at that. There could be a giant air bubble in the system. When the engine is cold, raise the hood. Take off the radiator cap. Start the engine. Turn the heat on, all the way up, full blast. And keep adding coolant every time it drops below full. Do this for 10 minutes.

Also, be sure that your cooling fan is running. Your truck has [in my opinion] an unusually complex cooling fan, being basically a combination of an electric fan and an old school clutch driven mechanical fan. All I can say is be sure that (1) it's running when the engine is and (2) that it speeds up as the engine warms up (according to AllData, it should run faster as the engine warms up). If it's not working properly, it'll obviously cause the engine to overheat.

A scan tool isn't going to tell you anything about an overheating problem unfortunately. The only way it could help you is if a coolant temp sensor is giving information that's well out of it's acceptable range.

Definitely use the heat not the AC. The heat uses coolant to warm up the air by using a blower to push air through a miniature radiator (aka the heater core) with warm coolant running through it. If the car got low on coolant, then air got into the system, even that part of it, and needs to be bled out.

If this doesn't solve the problem, we'll have to look at the possibility of a blown cylinder head gasket.

As a matter of fact, check your engine oil before doing much to it. If its milky/muddy, then go no further, you almost certainly have a blown head gasket and I'll detail more tests to confirm

Turn the temperature all the way up. That's the heat.