98 seville air suspension problem / question

I have been having problems with the rear air suspension in my 98 Seville. The dealer recently replaced (or said they did) the compressor, at at cost of over $800. 1 month later, it now rides like a lumber wagon-again- especially on the driver side. I hate to put more $$ into it. Probably an air ride shock on the driver side? How much $$ will that run in to? Or- should I just go to the replacement 'passive' systems?