Time for a new thermostat at least. Thermostats are a very common problem with Explorers. The TC is the transmissions torque converter that also has an internal clutch. It normally has a slip of around 300-500 rpm, but can lock up to incrase efficiency in third and overdrive. If the engine is overheating, I think it unlocks to increase engine rpm for better cooling. This is probably causing increased pump pressure in the transmission and causing it to leak more. At higher rpm slip (it can keep slipping up to about 2,000 rpm stall speed), the TC actually multiplies the torque of the engine. That is why you see higher towing capacity with an automatic with the same engine vs manual.
There are other things that can cause overheating like the fluid clutch for the fan. With the engine off, try spinning the fan. Should be fairly stiff. There are also clogged radiators. This is something you should take care of. These heads don't like to be overheated.