Try to stop just using the emergency brake with one had on the release handle. It would rule out the front brakes if there is no vibration.

Edited: On second thought, I'm not sure if the emergency brake controls the pads or just the shoes on the inside.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles