Being an 06 I had to have a Vin # of a medium duty that had an active 6 speed. Those vins would be from a mid year 07 to present. The vin # that I used was from an 08. I found a tech that had a GM diagnostic tool called a Tech II and he hooked into the OBDII port and selected the TCM in the truck and replaced my vin # stored there with the vin of the 08. It took about 10 minutes to replace the tables and activate the dormant sixth speed. In 06 all the pickups got the 6 speed but it wasn't until mid 07 that the medium duties got it. Most probably 05s and before won't have the capability to be reprogrammed just with the TCM but can have the valve body and solenoid rail replaced along with a programmed TCM of the 4th generation series installed and then your set albeit at some expense. Good luck
Denny