Will there soon be a Sierra HD AT4X? GMC appears to be ready to launch a heavy-duty version of the much anticipated off-road Sierra and we’ve got photos of a prototype testing these new upgrades.

Our spy photographer reports:

Judging by this latest prototype, it appears that the GMC Sierra HD’s 2023 update will bring with it a more serious off-road version, signaling a possible expansion of the ZR2/AT4X-spec capability into GM’s heavy-duty truck space.

Rumors are flying of a program within GM, actively testing a multitude of upgrades to go beyond GMC’s Sierra AT4 HD, and create a new AT4X HD variant. Engineers have reportedly been testing multiple shock options, swapping between upgraded versions of the traditional Rancho shocks, and robust versions of the Dynamic Suspension Spool Valve (DSSV) shock technology currently found on the light-duty versions of the Chevy Silverado ZR2 and its GMC Sierra AT4X brother. Now, with the appearance of this new prototype, it appears that this program has advanced from mere internal evaluation to the point of testing on public roads—not an insignificant step within GM’s testing protocols.

This new test truck displays a number of attributes that are consistent with the range-topping off-road variants in GM’s midsize and full-size pickups, and represents a likely step above GMC’s current Sierra AT4 HD trucks, moving into AT4X HD territory. Among the visible upgrades, we see:

Stripped-Down Front Bumper For Better Clearance
On the current GMC Sierra AT4 HD, the front skid plate is nearly invisible, as it is obscured by the truck’s rather low, chunky front bumper. One look at this new prototype’s highly visible (and beefed-up) front skid plate tells us that this new bumper design has been minimized, removing unnecessary, low-hanging bits like chin spoilers or wind diffusers. Much better ground clearance and an improved approach angle are in place for more serious off-roading.

New Tires Consistent With Other ZR2/AT4X Trucks in GM’s Current (and Future) Lineup
While the current AT4 HD is fitted with either Goodyear Wrangler TrailRunner A/Ts, or Goodyear Wrangler Duratrac tires, this new prototype is fitted with Goodyear Wrangler Territory MT tires. The Territory MTs are found on the recently-released Silverado ZR2, and have also been seen repeatedly on next-generation Chevy Colorado ZR2 and GMC Canyon AT4X prototypes. It appears that GM engineers have tried to scrub away Territory MT lettering on the tires, but the designation is still slightly legible upon zooming in. The Territory MT’s distinctive sidewall tread blocks are also unmistakably present on this prototype’s tires, confirming just what rubber is fitted to this HD test truck.

Disguised Wheels, As Seen On GM’s Other Specialized Off-Road Truck Prototypes
This Sierra HD off-road-focused prototype has camouflaged wheels, just as we’ve seen on some of the latest, more extreme Silverado1500 ZR2- and Sierra 1500 AT4X-based prototypes (which also get Territory MT tires). The camo’d wheels on this prototype, however, show a proper eight-lug design for this HD application.

A Look At The Shocks
This prototype’s shocks are tough to identify with certainty, but they look different than the current GMC Sierra AT4 HD’s red-tubed Rancho shocks. On this prototype, the shocks may display the wider format consistent with the Multimatic DSSV shocks that are currently part of GM’s ZR2/AT4X formula. DSSV shocks are not typically thought of as being robust enough to handle the traditional, HD-level loads, but rumors have suggested reports that DSSV shocks have, indeed, been tested on GM’s refreshed HD prototypes. It may be that the AT4X HD may be willing to give up some of the traditional HD capability in exchange for more off-road prowess. There have also been rumors of Multimatic developing a new generation of upgraded, HD-spec DSSV shocks, which would conceivably require less of a compromise between baja-busting and traditional HD-level attributes.

Whether or not this latest GMC Sierra HD prototype is fitted with DSSV shocks, it seems clear that GM is working on a new AT4X HD variant with a more dedicated off-road mission. Now that a prototype has been caught on public roads, we’ll be on the lookout for similarly-equipped test trucks, to dig deeper into its suspension details, and to see if something like it is also in the works for the Chevrolet brand.