Excursion overheats&leaks tranny fl towing going 50,uphill

I towed my similar size TT with the 7.3 and what I think is the same tranny you have. Was told by several techs and web sites that 125,000 miles is about the life span. Replaced with rebuilt unit and installed a 7 clutch billet torque converter. It seemed to shift and run smoother than the OEM. basically, you may have just hit the end of its expected life. RIP