2005 Chevrolet Trailblazer Running hard around 50 and 55

My trailblazer is driving me nuts! The trailblazer seems to be missing at around 50 55. The car shacks and then will smooth out. I have flushed and changed transmission filter. Flushed and changed diff fluid. Also cleaned throttle body changed air filter when I was getting a p0171 code. Reset check engine light after cleaning throttle body and filter. I have been driving it around for around a hour and check engine light does not come on. But still runs very rough at 55 and up a hill at 55. It's the wife's car and she has the kids most of the time don't know what else to do. Anything at this point would help.