Keep in mind the trucks you are looking at are rated to tow 12-15,000lb and that rating includes climbing grades. 2,600lb is nothing in terms of climbing power.
Even at altitude, I wouldn't worry about the power loss with just 2600lb in the back. Sure, you will burn some fuel if you want to keep it at 70mph up a grade but considering your average gas engine starts with north of 350hp, even if it loses 25% (equivalent to around 8000ft altitude), that still leaves you 260hp, which should be plenty to keep the speed up with such a light load.
