IMO a new diesel truck is a '13 model.
If you choose a 2500/3500 SRW Ram with the Cummins you get a 3.42 only which can tow up to 17xxx lbs depending on truck configuration. If you want a 3.73 or the 4.10 gears you will have to move up to the 3500 DRW that has up to a 30000 lb tow rating.
With the GM and the Dmax/A you get a 3.73 only for up to a 22300 lb tow rating depending on truck configuration.
The F250/F350 SRW Ford has up to a 16700 lb tow rating with a 3.31 and 3.55 axle ratio. If you want a 3.73 or the 4.30 gears you will have to move up to the F350 DRW which can tow up to 24500 lbs.