That was the one thing about the first Dodge I purchased in 2010...Had to get a 4:10 to get GVWR up to around 24,000#...same with the 2012, 4:10 took it to around 25,000#, if not mistaken. My 5er has weighed anywhere from 15,940# to 16,300# (loaded) and has a pin weight of 3340 to 3400#...but with the 13', it's a moot point...I stayed with the 4:10 because I do 99% of my towing in the mountains..GVWR is 14,000# and GCWR is an inane 37,000# or something like that......
What's the actual GVW on the OP's 5er...that's what one needs to know. The Cummins can do it and so can the 68RFE tranny....guess the rest is up to the OP.