Sorry I dont have the exact details right now for you because Im at work, but there is a bulletin for the transmission to be flushed 3 times and be refilled with synthetic. Had the same symptoms (always around 14-1600 rpms) and this was a temporary fix for about 10,000 miles. Thats my guess if you have the 8 speed.
Wow i havent been on this site in a long time. My sig pic is waaaay outdated. I have a 2015 Denali 6.2L now