GMC could be looking to expand its Bison lineup with a hotter version of the Sierra AT4X.
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"We caught another one of these extreme Sierra AT4X prototypes running on its own, giving us a better look at the upgrades in store for the uber AT4X variant. We don’t know what it will be called—the Bison nomenclature is likely exclusive to the Chevrolet brand—but we can see that GMC will address the rather low-hanging front bumper on the standard AT4X with a proper high-clearance bumper with cut-outs similar to the Silverado ZR2.
A stout front skid plate is visible—likely the same hot-stamped boron steel found on the Colorado ZR2 Bison, providing a very strong to go with its vastly improved front clearance. The skid plate protection will extend across the truck’s entire underbelly, fully protecting the vulnerable bits.
That new high-clearance bumper will finally allow for larger tires—at least tall as the standard ZR2’s 33-inch rubber, but rumored to be new 35-inch versions of the Goodyear Territory MT for optimum ground clearance. The Silverado ZR2 Bison and its AEV-fettled Sierra AT4X counterpart are expected to retain the same Multimatic DSSV dampers as the standard ZR2 and AT4X models."

Is This A GMC Sierra AT4X Bison Being Tested? - GM-Trucks.com
Is a GMC Sierra AT4X Bison model on the way? GM’s cooperation with American Expedition Vehicles (AEV) is set to expand to GMC models, as the Sierra 1500 AT4X
A stout front skid plate is visible—likely the same hot-stamped boron steel found on the Colorado ZR2 Bison, providing a very strong to go with its vastly improved front clearance. The skid plate protection will extend across the truck’s entire underbelly, fully protecting the vulnerable bits.
That new high-clearance bumper will finally allow for larger tires—at least tall as the standard ZR2’s 33-inch rubber, but rumored to be new 35-inch versions of the Goodyear Territory MT for optimum ground clearance. The Silverado ZR2 Bison and its AEV-fettled Sierra AT4X counterpart are expected to retain the same Multimatic DSSV dampers as the standard ZR2 and AT4X models."