5th and 6th gears are both overdrive. My '12 5 liter has the 3.55 rear axle and I tow 9500 lbs. I lock out both overdrives and let it roll. It gives good power up hill and decent engine breaking down hill. In the mountains of WV, NC, SC, I can set the cruise control and watch it do it's magic.
While 4 wheelers are hitting their brakes going down, mine, while towing, downshifts and needs no brakes. Uphill it seldom needs to shift. On the flats of NW Ohio, Mich, IN, 4th gear gives me better gas mileage, an extra 1 mpg at 65 mph. It allows the engine to operate at a speed that is more efficient under load.