To re-apply my earlier thoughts, I think it is the bushings or the kingpins if alignments, centramatics, shocks, and the correct tires don't solve the problem. Spend the few dollars and have it put on a suspension machine. It will find this problem. This machine is not a alignment machine. And speaking of that, make sure when doing alignments, that you are doing the whole truck and just not the front. Back axle could be out and you wouldn't know it. If the vibration is in the steering wheel, it is the front end. If it is in the floor or seats, it is the rear axle.
In addition, rubber neoprene bushings are used on both class 7 and class 8 trucks. If a heavy suspension is ordered, they only add a leaf ect for the springs. They don't change anything on the bushings. I don't believe brass bushings are used by any of the manufacturers when the truck is new except Western Star. Don't take the WS as fact, I can't remember if I read that somewhere or someone told me.
Seems to be a problem that keeps coming up. I listed what it took to get mine fixed. If someone else corrects the problem differently, let everyone know.
Davekc