GMC revealed the 2016 Sierra All Terrain X, a hardcore off-road package exclusive to the Sierra 1500 crew cab that builds upon the standard All Terrain trim with serious trail-ready upgrades. Available only in Onyx Black, the Sierra ATX features a 2-inch leveling kit, 18-inch gloss black aluminum wheels with Goodyear Wrangler Duratrac all-terrain tires, a bed-mounted sport bar with integrated LED pod lights, and unique red "Sierra" script badging on the tailgate. The dramatic press photography c