Some quick shots and copious use of flash show what is most likely a tank for diesel exhaust fluid and a diesel particulate filter, both necessary equipment for any modern diesel. It's unclear what form factor that engine will come in, though GM has a number of engines to choose from, including the 2.8-liter inline four that's found in the Colorado and Canyon, though that may be a little light duty for the Silverado and Sierra.
Other rumors surrounding the trucks suggest a move to an aluminum cab while retaining a steel bed. That seems like a reasonable move for GM, even if the company was attempting to shame Ford for its move to aluminum a few years ago. Look for more news as early as the Detroit Auto Show in January.