At altitude, the diesel should be much better even though the rated HP isn't much better on paper.
Isn't the top of that pass around 11,000ft. With a naturally aspirated engine you lose something like 3-4% per 1000ft above sea level, so your current truck is probably putting out the power of a smart car near the top.
The turbo on the diesel will result in negligible loss of power at altitude. Of course there is no free ride, it's more expensive up front and if you do 70mph up that hill, it will be burning fuel fast.
Of course, you have to decide what is acceptable. It's been a couple years but 45mph is probably faster than the semi's so you aren't creating a backup unless you move over into the fast lane.