I guarantee you that the hp and torque curves in a C12 on highway is completely different then a C12 in a dozer. Electronics make that change even more noticeable, but even a completely mechanical system would have a different curves.
As for the two speed rear end. I can't even begin to explain how bad of an idea that would be. an auxiliary transmission might be more applicable, but a two speed rear-end is just a recipe for disaster.
UVM SBA 2012