That was probably a GMC Sierra Vortec MAX truck. They had the word Vortec in small print and then MAX in big red letters on the doors (they didn't say Sierra in that location).
The Vortec MAX package was introduced in 2006 replacing the VHO (Vortec High Output) from the previous two years and indicated the LQ9 6.0-liter V8 engine among other options. I believe they discontinued the MAX badging a few years later.
Here's an example of one: http://cgi.ebay.com/ebaymotors/07-Crew-Cab-Z71-4x4-SLT-6-0-Vortec-Max-NAV-Sunroof-DVD-/160522349285?pt=US_Cars_Trucks&hash=item255fe0aae5