could be the actuator on the front axle. it just takes a big wrench or even some big channel lock pliers. and there is a wire to disconnect before removing. remove it and then replug in the wire. engage the four wheel drive and see if the actuator shaft comes out as the gas inside the actuator expands.
The 2009 GMC Sierra-2500HD has rear wheel drive.
The 2013 GMC Sierra-1500 has rear wheel drive.
The 2003 GMC Sierra-1500 has rear wheel drive.
The 2005 GMC Sierra-2500HD has rear wheel drive.
The 2007 GMC Sierra-2500HD has rear wheel drive.
The 2012 GMC Sierra-2500HD has rear wheel drive.
The 2009 GMC Sierra-1500 has rear wheel drive.
The 2012 GMC Sierra-3500HD has rear wheel drive.
The 2011 GMC Sierra-2500HD has rear wheel drive.
The 2010 GMC Sierra-2500HD has rear wheel drive.
The 2001 GMC Sierra-2500HD has rear wheel drive.
The 2009 GMC Sierra-3500HD has rear wheel drive.