There is a green telltale light in your instrument panel that lights up when MDS is on. It's controlled through a menu in Uconnect settings, you can enable or disable the telltale. That might help you if you haven't quite gotten the feel of the MDS just yet.
When you first get up to speed, Press the - button on the gear selector so 8 is displayed on the dash. This will keep the MDS from turning on. This way you will know if the vibration is from the MDS, or something else.
I've noticed the same. I'm guessing the ECU is calculating the engine load by rpm, air flow, and fuel consumption? Once it goes above a certain point defined by the engineers, hp needs override economy?