I've gone through 4 pickups in a year, a '90 3/4 ton chevy, a '04 duramax, a '95 dodge with a v10, and currently an '01 2500HD. The duramax was the nicest but was a basket case and every time I looked at it something needed to be fixed. I'm happy with the '01 2500HD with the 6.0 gasser, it will do 35-40mph pulling up the passes (I currently live at 9,300'). I bought the diesel for the pulling power and it was nice but I don't really see it being necessary in the midwest. I can still pull 70mph + on flat ground and with thicker air it would be even better. If you do go diesel stay away from the '07 emissions and newer trucks, they can be a huge headache.
Any 3/4 ton gasser should be plenty, a 1/2 ton in the midwest will even do the job. My '90 light duty 3/4 ton (1/2 ton with a bigger rear axle/ bigger rear brakes) with a 350 did the job just fine and was paymentless.
An option might be looking for an older crew cab gasser that you can pay cash for and then something decent on gas to drive when the truck isn't doing truck duty.