Selecting diff gears is about what RPM do you want for each transmission gear?

With your W56B and your current gearing and 34” tires in 4th gear your engine is turning 2975 RPMs at 70 MPH.  In 5th gear it’s turning 2380 at 70 MPH.

What is your peak torque at RPM now?

What would be the peak torque at RPM for a diesel swap?

Highway travel is my consideration.  On my 85 standard cab, I ran 33’s with stock 4.10 diffs.

5th gear was not usable for highway speeds.  In 5th gear my RPM was 2338 at 70 MPH – about 1,000 below my peak torque for my 22R.  I had to be in 4th gear at 2922 RPM at 70 MPH.  If I ran at 80 MPH, my RPM was 2672 in 5th gear – just at the beginning of my peak torque range and only usable on the highway with no head wind and on the flat.

As emsvitil said, off-road in 4-wheel drive, the dual cases will handle gearing requirements for practically any wheeling conditions.

Gnarls.