I have a 99 suburban 1500 4x4 and it has a wild roar/vibration in the 60 to 70 mph range. I've replaced the tires, had them balanced and checked again. Did alignment and finally took it to sears to see if they could figure it out. Guy says it's the front diff. Not sure as I drained all the fluid and was dirty but no metal. Perhaps wheel bearings? I love the truck but damn I'm perplexed.