I agree with much of what has been said. No one is "telling you what to do." I surely am not, but you could make a nice visit to Yellowstone and Grand Teton in that time frame. I would suggest spending most of the time in Yellowstone, but maybe a short move down to Grand Teton for a day or two.
And on the way from Michigan (over 1500 miles) you could swing up by the Black Hills in South Dakota which would not be far off your route. So Black Hills, Yellowstone, Grand Teton sounds like a good focus.
It would add three travel days just to go down to the Grand Canyon. Yes, it could be done in two very long days (for me at least), and that does not allow any time to visit any other attractions along the way.
I think you are going to have a good trip, with or without the advice of us old travelers.
