You most likely need a new conductor plate in the transmission. It's the part that connects all the solenoids in the transmission to the connector(above the valve body). There are two speed sensors on it and one of them is bad. This puts the transmission into limp mode. You will also need to have the code cleared @ a dealership, generics won't remove the code. You can probably do it yourself for around $200 plus whatever it cost to have the code removed.
The mercedes forums have detailed info on replacement of the conductor plate.