It sounds like a dumb question, but how do you know you have sufficient coolant? I assume your checking the radiator level and not just the overflow container.
Because of the overheating and no loss of coolant I'd consider the thermostat first.
A worn bearing will cause noise, but unless the impellers have been seriously abused (no examples I've seen on Explorers) it will still push water. Generally speaking the water pump will leak when it goes bad.
Two common diagnostics follow. You can combine them with the engine cool and then hot. Be careful around moving parts.
With the engine cool start it and pinch the top radiator hose. After the car warms up, squeeze the hose again, it should be considerably stiffer, that's the thermostat opening up and flowing water. If not stiff the thermostat's bad.
With the engine off and the car cool, turn the fan blade by hand it should move easily. With the engine off and warm, turn the fan by blade by hand again it should be noticably stiffer. If not bad fan blade clutch.
From here it gets more exotic. Let us know how it works out.
