[beautimus]

ok, just for giggles and grins heres my guess. and the logic (faulty or not) that goes with it.
heres how i would trouble shoot it.
since, you would know if you have a major coolant leak we will assume that there is none.
1 let the truck set over night so its completely cold and the radiator is depressureized.
wash all the bugs and dirt from the fins and check to see if the fins are loose from the tubes of the radiator. the little fins are actually soldered to the tubes at the factory. if they separate,
the radiator is not gonna cool worth a crap, and needs to be recored or replaced. if it looks ok keep going.

2. pull the radiator cap off (engine off and cold)
fill the radiator with water (at this point i am gonna assume i gotta tear something apart, and no need to waste expensive coolant.) turn the heater on , i want to circulate weater through the heater core to get any air out of it too.

3. start the engine and let it idle, add water as needed. the coolant level should drop but not start to circulate untill the engine warms up enough to open the thermostat. if the water starts to circulate through the radiator (moving fast side to side)
immediately upon start up then the thermostat is stuck open.

3. let the truck come up to operating temp, (cap still off)
when the thermostat opens the water will begin to circulate
through the radiator, water level should drop a bit, add water just a bit at a time (don't add a lot of cold water at one time to a warm engine, bad stuff could happen)
watch the coolant for small bubbles that don't stop coming. some bubbles is normal but a lott of bubbles that keep coming constantly coming to the cap area is bad. could be a cracked head or a head gasket going south letting compression gasses enter the water jacket.( check for coolant smell in the exhaust to verify.)
4 put the cap on and let it run for a few more minutes,
the radiator should begin to pressurize. squeeze the radiator hose to see if its pumped up. if it just pushes stuff out around the cap, replace the cap. the radiator has to pressurize in order to raise the boiling point of the coolant above 220 degrees.

5. if you don't know the history of this radiator, somebody could have put bars leaks in it, it will stop a leak, but it also lowers the efficiency of the cooling system, go get a couple of cans of radiator flush and flush the radiator with the heater on, clean out the heater core too, it will work better.
if it still overheats, i would start looking at
the belts and hoses and make sure there in good shape.
thats my guesses,
tony