Since you have already had the wheels rebalanced at a different shop, it is time to find a 9700. I disagree about this being done ONLY by an MB dealer. There are plenty of 9700's around. To find one go to: GSP9700.com There is a locator on this site that will tell you where there is one in your area.
Remember, when you are replacing/repairing such things as suspension components and steering couplers, you are not going to the source of the vibration, you are only correcting things that are making the vibration slightly worse. Don't get me wrong, all these things should be in good shape. But, you could easily spend a fortune getting all these other things in perfect shape, but will still have a vibration if there is a source of vibration.
By far, the most common source of vibration is the wheels and tires.
You did not say what kind of tires you have.