I too just purchased a GMC Canyon (sister of the Colorado that you purchased) and it feels to me like there is some vibration present as well. How did they determine the tires were out of round, just a speed balance check or something? I am taking mine in to have it checked as well - I don't want to over-react, maybe the off-road suspension makes everything bumpy - but it feels like there is more bounce when traveling at a constant 45-50 MPH than there should be on a flat smooth road. Do you also have the off-road suspension in your truck?