Excursion overheats&leaks tranny fl towing going 50,uphill

Is the tranny fluid leaking from the round black plug an the bottom of the transmission bell housing? If so, it could be the torque converter seal. This seal can leak when towing because of normally higher transmission temp and then be perfectly fine when not towing. If this is the problem, it is still expensive to repair (the part is about $5 so it is all labor charges). Sometimes, the torque converter gets grooved by the seal and also needs replacement because of that. Best of luck.