Vibrations at speed are often caused by moving parts that are out of balance, not loose suspension parts or vehicle alignment. So if you have a vibration at speed, look to things that spin that might be out of balance. In fact, with just about all vibration issues, look to rotating assemblies for the cause. Once they hit a certain resonance, the vibration occurs, so be observant in your search for the cause. At what speed or RPM does the vibration happen? Does it coincide with a noise? Any clues you can find will help you determine the cause of a vibration.