I concur with 420 SEL. I had a terrible time getting a proper wheel balance and alignment at a local, well-regarded tire shop.
I started a post called 'bad wheel balance or bad tires" several weeks ago, and I got a lot of good advice from other members, Benzmac, Stevebfl, and other MB techs.
You can read all about tire balance and alignment issues on this thread.
What's clear is this - you need to go either to the dealer or an MB-specific independent that has the right alignment tools. Hunter is the company that makes popular alignment machines that many shops use, and they work with MB for theirs. There are special tools that are required to a proper alignment on an MB, and only the dealer or a good independent will have these.
Last, check the website
www.gps9700.com . They make the balance machine that properly balances a tire. Enter your zip code, and a whole list of shops should be listed. The MB dealer near my house uses this machine, and the difference in ride was astonishing.
Fwiw, worn out tie rod ends can also cause this kind of tire wear.