Could be a possible driveshaft problem...like maybe not weighted correctly or lost a weight possibly. I would have them check that out. What makes me think about that is when you say HIGH speeds. I had a "tire out of balance" and mine would "shake" like what you are describing. I took to independent tire dealer and he said that he couldn't get it too balance. I had him note that on the receipt and took it Chevy dealer and they replaced the rim.
Just something to think about.
