Although the U.S. truck market is amid a heavy decline, there are still those that need the capabilities of a pickup truck for various reasons. Because of this fact, General Motors is hard at working developing its next-generation of heavy duty pickups and our spy photographers just spotted a GMC Sierra Heavy Duty mule undergoing some real world testing.

Making no effort to disguise itself, the 2010 GMC Sierra 2500 in these pictures is running with GM’s updated 6.6-liter Duramax V-8 under its hood. The Duramax is expected to receive a significant increase in horsepower and torque next year (well over 700 lbs-ft of torque, sources tell us) as well as meeting full Tier 2 Bin 5 federal emissions rules - the toughest clean diesel standard in the world. The Sierra’s grille has been toned down (from four horizontal bars to three with a squat honeycomb pattern running the length of each bar) and its front bumper has a larger inlet for the Allison transmission’s oil cooler, like the 2010 Chevrolet Silverado 2500 we recently spied.

If you look closely at shots of the Sierra’s right front passenger side you can see what we believe is a temporarily placed urea tank attached to the frame for testing out the new emissions system. Urea fluid is injected into the exhaust stream to reduce nitrogen oxide levels. When it reaches production, the urea tank will have to be refilled periodically during routine maintenance and service.