I purchased a certified used 2008 silverado 2500 diesel. since owning the truck we have had issues with vibration beginning at 45 mph through 75 mph. Has been back to the dealership many times and they suggested new tires and rims. Did that, still vibration exists, though not as bad. The truck only has 59K miles on it. What else could be causing this? We have had the U joints checked and drive train by said dealership, but they say no problem found. We did discover today a worn out left rear shock. Do not hink that would cause that type of vibration.