There are multiple wheel speed sensors and there is no way to determine which sensor is defective with out a scanner. If you are going to keep this vehicle and do your own repairs I suggest you purchase an iCarsoft Mecedes II scanner. You will find it invaluable. I would also suggest a digital manual. They are inexpensive in ebay. By the way, not only can a wheel speed sensor cause your issue, but also an ABS brake module may also cause the same symptoms. Also, if you do replace a defective sensor, you will have to clear the fault codes - which requires a scanner or a trip to the dealer or a Benz shop.