My 2003 trailblazer ext has 129k on it and has never had the transmission serviced. It shifts fine, no issues. I was talking to my mechanic about changing the fluid and filter and he said you might want to think twice about doing it at this point. Not that it has happened with a trailblazer, but he had some vehicles transmissions go bad after they were serviced that had high miles with never being opened up before. he sad sometimes it's not worth opening them up.