I'm having the same issue with a GMC Denali 3500 DRW. I replaced driveshaft, shocks, front disk's, front calipers, steering dampener, transmission fluid, another set of new Michelins, and have Centramatics on order. Frustrating as heck to have it vibrate from 66MPH to 90MPH, below and above its fine.
Please post solutions if there is any.