Flush the transmission fluid. Maybe install an even larger trans cooler. Those are cheap changes before replacing the vehicle. 240* is a little high, but you tracked the sound/vibration to 215* which is in line with your towing conditions. Also, you may want to force the transmission to stick to 2nd gear. Shifting and more specifically unlocking the torque converter drives up the heat to 230+.
If you are looking at newer tow vehicles and continue to tow at those elevations, I highly recommend the Expedition with 3.5L Ecoboost. Forced induction will not lose as much power at those elevations and the peak torque in the 2500rpm range will not need to shift as much.
