Instead of taking it to the stealership, find a reputable authorized shop that works on MB's and take it to them. This is exactly what I did and sure enough turned out to be the right rear wheel speed sensor. I bought the part at MB of Austin and replaced it in about 10 minutes.
Cost me 45 bucks for the shop's time and I forget what the part cost exactly but I'm pretty sure I got away for under a 100 bucks.

Just my Dos Centavos