Need to pull a large boat, horses, construction equipment or a luxury travel trailer? You might need something more than a 1500 Series pickup. Step on up to the GMC Sierra 2500 HD. With the Duramax 6.6-liter turbodiesel V8 churning out 910 lb-feet of torque, it can tug a lot of kit. Tom Voelk checks out the top trim Denali model.