I agree with respect to payload that a heavy half or 3/4 would handle the load better, but the engine itself is more than capable. Don't be afraid of letting a gas engine run! These are not the big blocks of old, but new high tech OHC engines, and they generally rev to make power. CAFE requires auto makers to eek out every last bit of mpg, so the default program for every car/truck is to get into high gear as fast as possible, with double overdrives of 0.69:1! The only time most people see 3500 - 4000rpm is if they step on the gas from a stoplight, and that is for half a second. Now put a heavy box with no aerodynamics behind a truck and it has to work. Since many people generally are not used to the way this sounds they sometimes feel it is bad for the engine, I assure you it is not! At 3500rpm the 5.6 is just entering the meat of the powerband! It will run at 3500, 4000, 4500, with no issues, it is designed to do this, in fact, peak torque is around 4300. It is actually better for it to rev faster. The force impulse is lower on the internals, the cooling fan is running faster, the torque converter is usually able to remain locked in a lower gear. It will also feel more responsive if you have to add throttle, the engine is already there, it doesn't have to lag and downshift.
I often pull our trailer up and over mountain passes and usually manually select a lower gear before the truck wants to downshift, I will keep the rpm in the 3500-4500 range to keep the engine in the fat part of the powerband, and have no trouble maintaining a reasonable speed. I will also lift off the throttle and maintain a speed and gear on occasional flat parts of a grade rather than sifting up then back down. Recently pulled Towne pass going into and then coming back out of Death Valley. Coming out the grade goes from 0ft to just under 5000ft in about 17miles with sections ranging 7-9% 2nd gear 45mph, no problem. Truck has enough sound insulation that it isn't that noisy.
