The ride on my car has become an issue. It seems to ride very stiff. Every time I hit a bump I hear a muffled hollow sounding thud in the rear of the car(like the shocks/struts are bad or something). Also at around 55-70 mph it vibrates up and down. Is it the shocks? If so is there a passive suspension system available for my model because oem replacement costs up to $3000! Anyone have any ideas or a similar problem please help. thanks.