I have a 2007 gmc with about 35k miles and I have 2 friends one with a 2008 and another with an 07 like mine we all have the same problem and mine seems to be the worst of course. After getting a nelson tune we all experience slippage from 1-2 gear shift ? My truck will shift out of first, Well try to shift and drops to about 4500 then slams back to around 3600 when it finally shifts all the way very strange. My mods on this truck are comp 212/218 low lift,Circle d 2800,LT headers, nelson tune and AFE cold air. My 2008 has more mods and a 408 and it does not slip ? But this truck was tuned on the dyno at TTP.