Never owned a dually

Choosing the final gear ratio is really simple; you determine your loaded trailer weight and follow the manufacturers recomendation unless you feel you know something the engineers who designed the truck don't know. Personally I might choose a 3.42 even for heavy trailers. I would lock out 6th gear when towing. A 3.42 rear end puts the engine at 1725 rpm at 62 mph in 5th gear. Hit a hill and one downshift brings the rpm up to 2240 where the engine is capable of making about 350 hp. If that can't get the job done the Aisin will drop to 3rd and at 58 mph the engine will be capable of putting out its peak hp of 385. And the Cummins should just be capable of pulling a 32000 lb combination up a 4% grade at 58 mph.