In all seriousness, the suggestions on U-joints might be on the money. Prior to doing my front axle U-joints, I had a slight vibration at 55-60 mph, not much and not above or below. I always thought it might have been tire wear, but it went away after I did the u-joints. Could it be the hub units are seated better? Probably not, so it might very well have been the u-joints. Did not touch the driveshafts at all, just the front axle u-joint replacement.