Install a new high-flow thermostat at the factory recommended heat rating (I think 195). That will actually help, not hinder cooling.
With no thermostat, the water actually passes through the engine too fast to absorb heat, it can also have an air pocket in the system that is hindering the ability to take away heat.
How is your water pump? They do fail and the truck will boil the radiator when they do. Mine did just recently. It failed because I had plugged the radiator fins with silt from an off-road run, and it actually sucked the bearings right out of the pump. A new pump isn't much money and it is FAR cheaper than replacing a cracked head from overheating.
Next, check out your transmission. Is it overheating? Remember, it cools itself by dumping heat into the radiator. That is fine, but if you are using it hard in a high-heat area, you will most likely need to install an auxillary cooler.
Then, check the engine with a good scanner (not a code-reader) to see what it is ACTUALLY doing temprature wise -- and also if there are any cylinders that are mis-firing, which is an indication of a head gasket or other leak in that area. A good shop will also have an exhaust sniffer that can be used on the anti-freeze to detect exhaust gas leaks through a bad head. Of course, the old test is to open the cap, run the engine and see if it is boiling out of the radiator from a blown head or head gasket.
I'd also check to make sure that you don't have soft hoses. They can actually collapse and suck in, especially if you are not running a thermostat.
Are you running the recommended 50/50 blend of anti-freeze/water? Plain water does not cool as well.
When is the last time you changed the air filter (or cleaned it if it is a a cleanable one)? Is your engine running rich or lean from a bad O2 or MAF sensor? Are the plugs and/or wires bad? Brake dragging? Transmission slipping? Oversized tires without proper gearing?
These are the most common reasons for overheating... Assuming that you have a decent radiator.
