Does my usage warrant diesel? Cummins 6.7 with AISIN...

I have also been theorizing as to why they chose the "higher" 2nd gear ratio. My truck will do 70 mph in 2nd gear, even with the 4.10 axle. I wonder if they use that ratio to enable 2nd gear to be useful for highway passing or pulling grades with a load.

I know the tq converter doesn't lock in 1st gear so that (prevents excessive heat build up) might also be a reason they limit the rpm.