Cat c15 with 13 speed read end for middle ground speed and power

I don't think changing your rear ends will accomplish what you want, if your problem is maintaining better speed in the hills look to put more power in the motor. Think your money would be much better spent. What year and model is that C-15.

 

Have you considered using the splitter and dropping a gear here and there to keep the engine in the zone?
Just asking.

If you run one gear down at 1500 to 1600 rpm you're only a few mph slower than top gear at 1400 rpm, you have a steadier pull and longer legs to run out at the bottom of the hills in the big hole.

If you're pulling over 80k all the time or dragging a wind wagon I can see where you may have legit concerns.

Edit: second thought.
Going to a lower profile tire might give the desired result.