Check your spark plugs
 

2012 GMC Sierra 2500 HD 6.0L, 133,000 miles. I just changed plugs on my truck this past weekend, I changed them once before right after I bought the truck at 79,000 miles, so these had right around 53,000 miles on them. AC Delco Iridium 41-110 (old), 41-162 (new) plugs. There were no issues like bad (worse) mileage, mis-fire or hard starting just figured it was time to change them out, cheap maintenance item. The only thing I had started to notice was a bit of hiccup or hesitation under load when going up a hill in 6th gear but was fine otherwise. I was surprised at the condition of the old plugs and that it ran as good as it did. Owners manual says to change plugs every 97,500 miles.