My '99 S600 has suddenly developed a problem in the rear suspension. It rides like I have 50 pounds of air in the rear tires. The car rides so bad that the CD Changer is having a job keeping on track! At the same time the rear has developed a grunting sound on the slightest road irregularities. I remember reading somewhere that the hydraulic accumulators may need attention. Is this a rebuildable part? Any other ideas?