PTP,
I THINK this is the reason. Horsepower = RPM X Torque. Theoretically, it will always take the same amount of horsepower to move your truck + trailer + boats + stuff at say 70 MPH but you can vary the RPM and Torque to achieve that horsepower. The idea behind an OD is that the engine can normally more efficiently develop the horsepower to cruise at interstate speeds (without a load) at lower RPMs and higher torque. The problem is that when you add a load the required horsepower increases due to higher weight and drag. The RPM to go 70 MPH in OD does not change so the torque must go up. This causes two issues. The first is that the engine is probably not that efficient at this new RPM/Torque ratio. The second is that your transmission and the rest of the drivetrain can only handle so much torque before it is damaged. As torque in a gear box increases, the pressure/friction on the gears goes up. If it goes too high the thin layer of oil that is allowing the gear teeth to smoothly mesh gets squeezed out and then you have metal scraping on metal, which is bad. To handle more torque you either need a better gearbox (bigger gears = less PSI of pressure) or better oil/lubrication system. Where I work we recently increased the torque capacities of our gearboxes by 10% just by switching to a better oil. (synthetic instead of conventional) The heat a gear box generates is do to friction. As you transmit more horsepower through gear box it generates more heat. Also, as noted above automatic transmissions, slip under high torque and that generates heat too although it may protect your gears. Your gear box/oil has an optimum operating temperature. If it gets too hot it will cause damage. That is why a lot of gear boxes will have oil coolers. Sorry, to be so long winded and I may be wrong.