Where did you get the info about the only rear end being 3.73?
Maybe they changed it, but I thought 3.42 was their main go to gear now. My dad's '18 with the heavy duty (non 68rfe) runs crazy low rpm at highway speed. To the point that towing our 5000 lb boat it is constantly shifting out of 6th on any hill. Towing at 65 mph in 5th gear it runs about 1750 rpm with tons of power and actually gets better mileage towing the boat in 5th than lugging it in 6th. His has 3.42 gears.
You sure you got it into 6th gear? Or perhaps the transmission gearing is that different.