My wife's ML had this problem so bad that I refused to drive the truck. After looking into it, the seat frame had failed on the right side at the tilting shaft. Looked like a bad weld but the vertical metal flange seperated at the tilt mechanism shaft. The dealer in San Antonio actually stocked a replacement so I figured it was a known issue. I have no confirmation on that, but they don't even stock radaiator hoses so it seemed suspicious. They refused to comp the part so at $400+ so I pulled the thing apart and rewelded the pieces together. About one hour and no more problem.

Victor