stanley TILTON

I really can only explain it and hope someone reads this and they know. Its something it appears i only notice. 3 shops and endless people loooking at it. It started as small vibration only on interstate felt in pass. rear side. Now it will drive like its a tank every road different truck riding. concrete roads being the worse. It feels like its vibrating apart but then it will stop and feel like a new truck drives smooth for a wile then back to crap. replaced motor mounts,hubs,back shocks , universal joints still the same. any suggestions. If others drive it they dont, You have to drive long period and you will notice two different trucks driving. At 40 to 45 holding steady the rpms bounce up and down about 300 rpm . Makes driving in town tough.