if there is a body lift on it then you will need to cut your floor board if not then check the linkage on your shifter if that is good then something is wrong with your transfer case
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.