The friction generated between gears is a function of the force on the face of the gear tooth times the friction of the surface. Therefore, the higher force pushing on the gears, which goes hand and hand with more power, results in more frictional losses. To test this, drive your pickup around the block for 10 minutes at 25 mph, then measure the temp of the rear end. Let it cool (if it even got warm)and then get on the highway and stand on the gas for 10 minutes. Check the temp again. The higher temp indicates that more energy was lost when operating at higher horsepower.