Here's how I would figure out where the problem is:

1) Move the front tires to the rear and see if the vibration changes. If the vibration problem improves, swap each rear tire out for the spare and see if you can improve the situation further. This will help you identify a bad tire or out of balance tire.

2) Have them balanced again by an MB shop.

3) Have an MB shop do an inspection of your suspension and do an alignment if everything checks out ok.