Is is normal to have somewhat excessive vibration at highway speeds? I have drivin a few other vehicals, a chevy astro van, a bmw z3, a chevy van 30, plus a newer nissan murano and my sportie has a butt-numbing vibration. I realize the sportie is not the nissan and not the bmw, but the two vans are much smoother in the vibration department. I'm not talking about the truckie ride, it's vibration. Anyone suggestions?