F250 Transmission Question

I have a 2000 F250 with the powerstroke engine and auto tranny. The truck has 172K on the odo. Most of the miles are highway but now I only use it to pull my toy hauler, boats, and other trailers. I installed a temperature gauge on it back when I first started pulling my hauler and the temp never ran over about 195 or so, but recently the temp has started climbing up to around 210 or so even if I'm not pulling. I had the tranny oil and filter changed about 2000-3000 miles ago. and pulled the hauler home from about 6 hours out with no problems. Last time I pulled my hauler, I had problems with the tranny overheating and it even had fluid leaking out from what I'm assuming is the bell housing or torque converter (not sure exactly what it is).
Today when I drove the truck with nothing behind it the tranny gauge heated all the way to 210 or so again. My question is am I looking at a rebuild or could there be something else that is a problem? Is it possible that the torque converter is slipping causing the tranny oil to heat up.
Also just for info when I tow I DO NOT push the overdrive lockout button to turn it off. I was told that this was not needed on this truck. Thanks for any thoughts or suggestions.

Pick up a can of "TransTune" and use as directed. Then drain (including torque converter), clean pan, replace filter & Fluid. Improves flow & heat transfer function of Trans Fluid cooling coil! If temp drops while additive is in use, switch to synthetic at change. Far Superior over standard Trans Fluid.