– Moab, Utah

“We’re going to have to turn around.”

Not because this 2017 GMC Sierra 2500 Denali couldn’t tow 5,000 pounds up the approaching steep hill, but because the 26-foot trailer attached to it would never get around the tight switchbacks. Stu Pierce, GMC senior marketing manager, misinterpreted some directions on our towing test loop and now wants me to change directions on this rural two-lane road.

Doing so means making a wide arc over some muddy ground on the left of the road, and hoping I can get the truck and the trailer through more muck on the opposite side. Four-wheel drive activated, Pierce guiding from outside, I pull forward through the turf and cross my fingers we don’t get the whole rig stuck.

The truck slips sideways as all four tires spin, kicking up red clay-like dirt in every directions. Then they hook up, dig into the mud, and without much fuss at all, truck and trailer emerge unscathed, now facing back toward the small town of Paradox, Colorado, instead of up that hill. It’s a good, if unintended, taste of some of the capability available from the 2017 GMC Sierra 2500 HD.

Redesigned for 2017, the heavy-duty truck is tougher than ever. The big change is the new Duramax 6.6-liter turbodiesel V8 engine underhood. Codenamed L5P, about 90 percent of its parts are new compared to the outgoing LML diesel engine. Outputs rise from 397 horsepower to 445 hp, and from 765 pound-feet to a massive 910 lb-ft of torque. Those numbers are right on top of the ...