Are you SURE that the bearings are the problem. It is quite common for tires to roar so bad you can very easily convince yourself that the problem is bearings.

I have never had to replace rear wheel bearings on an MB even at ridiculously high mileage. My 240D has over 500,000 miles with the original bearings untouched as does my 300E with 260,000.

Make SURE that the problem is not the tires. I have even had tires that LOOKED perfect but once replaced everything was silenced again.

Good luck,