My 1999 RL (215k) is making a "shuddering" sound from the rear when going over rough terrain. I figure it is the strut(s) but my mechanic says the struts rarely have to be replaced, that it might be a mounting plate for the struts, which he says do have to be replaced more often. Bushings? Any other suggestions? I'd rather not replace the struts (major deal) and it rides very smoothly on the highway. Any other thoughts?