The first thing is to check the clearance between the round part of the salt shaker and the rubber mount above it. Two things to check -- evenness of the gap around the circumference of the mount and the height of the gap. In my 91 300SE, the gap around the left mount is not even and the car lists to the left. If the round part of the salt shaker is pretty much in contact with the cup part of the subframe into which the mount fits, the mount is pretty much collapsed. I don't know what the spec is, but less than 1/4" is getting thin IMO.
Hope that helps.