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.