I'm having the same problem with my trans. I've read online that these trans have a lot of problems with the input and output speed sensors.
automatic
If its a half ton automatic its a 4l60E if its a one ton automatic its the 4l80E.
The 2008 GMC Sierra-2500HD has a 6-speed automatic.
The 2014 GMC Sierra-2500HD has a 6-speed automatic.
The 2012 GMC Sierra-2500HD has a 6-speed automatic.
The 2008 GMC Sierra-3500HD has a 6-speed automatic.
The 2007 GMC Sierra-3500HD has a 6-speed automatic.
The 2013 GMC Sierra-2500HD has a 6-speed automatic.
The 2011 GMC Sierra-1500 has a 4-speed automatic.
The 2011 GMC Sierra-2500HD has a 6-speed automatic.
The 2007 GMC Sierra-1500 has a 4-speed automatic.
The 2012 GMC Sierra-1500 has a 4-speed automatic.