Got a 2013 GMC 2500HD Denali 6.6L LML, with 800 miles. The issue is as follows: highway speed (74mph), obtrusive rear vibration apparent. Two trips to dealer, to balance/rebalance tires, check alignment. Nothing found, told vibration from possible tire flat spot (sitting on lot for awhile). Any others with similar issue? I ask because I test drove 2014 model, and had similar vibration present at highway speeds. For the cost I expected a smother ride, this happens to be more than a beefier suspension effect. I feel less vibration when tire pressure reduced to 58/72 psi front/rear, from 62/80 psi tire pressure when took delivery.