I'd start by checking the clutch on the fan. They're known to go out. If you can stop the fan from spinning (please be careful -- use a stick or something) the clutch is bad.
Next on the list is a bad water pump, as some of the others have already said.
Any of the above stuff can easily be caused by an day on the trails, if your radiator gets plugged with mud (or even washed pretty good with silt). That will actually plug the air flow enough to such the bearings out of the pump and destroy the fan clutch... Ask me how I know...
Then, the thermostat needs to be checked -- if I had the water pump out already, I'd just change it. Also, replace it with a stock temp unit -- don't try to go cooler, the engine won't run well at all in the Explorer. You can go with a better brand of thermostat that flows better while open, but keep the heat range the same.
Finally, it could be a cracked head -- that is not an uncommon problem. The easy way to diagnose a bad head is to see if it is boiling the radiator by running the engine with the cap removed. It should be flowing and swirling, but not bubbling out of the radiator while running. Another test is the compression test. Pull all the plugs, screw in the compression tester to one spark plug hole, crank the engine -- record the number. Proceed to the next hole until all 6 are done.
Two low readings in holes side-by-side indicate a cracked head (generally). A single low reading could be bad rings or a bad valve, or sometimes also a cracked head.
Finally, one last test is a test strip that detects exhaust gas in the anti-freeze. Most good techs have this test on hand, or can use a 5 gas analyzer to check it out.