Everybody is anxious to jump in that back panel and put a new actuator in but it might be wise to get the Freon checked first. I redid all my air last year and it worked great. Then at Christmas, in FL, the A/C wasn't blowing cold. Turned out I lost most of the Freon through one of the fill/test ports (Schrader valve was leaking and the cap wasn't tight). I could see the dye all around the fitting.... I topped it up with an auto-zone quickie can and the truck was good for the rest of the trip. A couple weeks ago here I noticed the front was cold but the back was cool at best. I started poking around thinking it was the actuator but a mechanic friend said 'check the Freon level first.' My wife took the truck to work (GM dealership) and sure enough- it was down 1.5 lbs. They topped it up, couldn't find any leaks and now the 02 blows colder than my '13 sierra - front AND back.
Just might be an easier/cheaper thing to check first....