Automatic Locking Differential (ASD)
WARNING: This page is about a different car, the 1994 Mercedes-Benz C280 and 1994 Mercedes-Benz C220. However, it is still accessible from the selected car via links, so may be relevant.
Turn ignition off. Slowly open bleed screw on right annular cylinder. Annular cylinder is located on differential. Let hydraulic fluid flow until pressure is released.