gear ratio.

With 800-ish lb-ft of engine torque, you likely won't notice any difference between 3.31. and 3.55, or 3.73. You will probably be stuck with whatever it has based on tire/wheel size for 2011+ Fords. No one has complained about the power of any 2011+ diesel regardless of axle ratio offered. Pick the trim/options/wheels you like and tow happy.

3.55 or even 3.31 would be fine. If you ever think you are going to tow real heavy then I would get 3.73. You will save some fuel with the 3.55/3.31

If you do go diesel, gear ratio is less of an issue; yes, a numerically higher ratio provides more wheel torque, but the primary reason folks love to tow with diesels is they produce their torque at much lower RPM than gasoline engines...so, a diesel doesn't "need" low gears to get into the optimal powerband as a gas motor may.

Also, as powderman said, newer 5-and-6 speed automatics make rear end gearing less of a factor than with the older 4-speeds. Likewise the crazy amount of power these engines produce; with 800+ ft/lb of torque, lower gears just simply aren't as necessary as before.

My 05 has 4:10 gears which I feel is overkill in most cases when pulling about 10k. I will say on a 9 or 10 percent grade its handy but I don't often see that. Also as I understand the newer trucks have more speeds in the transmission which can change what is needed in the rearend. I feel a 3:73 is a good match.

I just bought a Ram 2500 Hemi with the 3.73. It has good power. Most diesels have a 3.43 in them I think.