91 350SDL rear diff question
 

I have finally lift my car up to see any problems underneath , and what i have noticed rear diff has some hydraulic lines are going into. The lines is tracing back to small reservoir at rear (it's like for holding some kind pressure). So I have a big fluid reservoir at left front (ZH-M fluid marked there), and I was reading it's for self leveling suspension, but it looks like it's not for it.

Any ideas?

I had one of those. Mine did not have SLS and did not have any lines running to and from the diff. Seems like it must be related to the sls but how I have no clue. Pics might help.

Your car may have had the SLS or ASR with the original 3.5L engine. When it was converted to the 3.0L, they may have done away with the SLS or ASR system and just capped the lines to it.

That is for ASD, which is the fancy Mercedes version of a locking differential. It is tied into the ABS system to detect wheel spin at the rear axle. When either or both of the rear wheels start to loose traction under acceleration, the ASD computer sends hydraulic pressure to the differential, and internal clutches "lock up" so power goes to both wheels regardless of the wheel slip, which improves traction. When the system engages, a flashing yellow triangle with an exclamation point in the middle will show on the speedometer.

I see. I will try to make pictures next time on lift.