First off, when your engine gets that hot you are dangerously close to doing damage and warping your heads, so I'd strongly suggest getting it fixed!
Usually in my experience when an engine overheats it's a failure/issue in the following order:
1-Water pump has gone out
2-Fan/fan clutch is bad
3-Antifreeze is low (which means you either have a leak somewhere or a bad head gasket letting antifreeze leak either into your crankcase or it's getting into the combustion chamber and existing your exhaust)
4-Radiator is clogged/gunked up (pretty unlikely unless you've done some weird stuff or put the wrong fluid in)
5-Rare cases the thermostat could have failed shut, they should fail open
First and simplest check, does your cooling fan come on, the big mechanical one when you are running the truck and it gets warm? It should pretty much be running at some speed when you start the vehicle and increase in speed/kick on fully when you're warming up. If the fan is working move on to the coolant/antifreeze. Check your overflow tank, if you're facing the engine it's the clear plastic tank on the front of your engine with the yellow cap, do you have reddish/orangish fluid in it? About 1/2 full? With the engine COLD, not WARM or HOT or you'll get a lot of hot fluid under pressure coming out, open your radiator cap (push down, twist to the left to release it), and look in. You should have fluid in there, look down the open hole with a flashlight. If you do have fluid in both, then the most logical candidate is a water pump failure. Normally water pumps weep through a small hole when the pump bearings go out, which typically can happen after enough age/wear. The water pump on our trucks is attached to the mechanical fan, shine a flashlight down there where the fan shaft attaches to the water pump on the front of the engine and look for leaking/dripping fluid or look under your truck and see if it's got any antifreeze under it. Grab the fan and see if you can wiggle it around ie the water pump could be really bad and everything wobbles. How many miles are on your truck? You are at the right age at least. You can tell if your water pump works if you warm the engine up to operating temperature and feel the upper radiator hoses, they should get very warm after your thermostat hits the set point and opens, your truck is probably at 205 degrees or something. It will be hot! If those hoses don't every get hot, and you have enough fluid, then it's a good indication your water pump is bad.
Since your truck cools down when you're moving, it doubly indicates a problem with either the fan or water pump, you have enough coolant in the radiator for cooling when the air from your moving truck moves over the radiator fins and some coolant is flowing due to the temperature differential in your cooling system.
Let's just hope it's that, occam's razor says the problem is usually the simplest thing, changing a water pump on our trucks isn't bad and shouldn't kill your bank account.
