Hi, Zee-car riding low in back/tire almost touching wheel well/cost to fix/

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Hi if anyone can please help , I just got a quote from Firestone over $900 to replace shocks on back of 97 Volvo--It has been riding very low and I was just adding air//a guy told me no- you have suspnsion prob/ he said i need to replace shocks/or struts...please help...I have another estimate for tomorrow, I want a fair price...any help appreciated. thanks Erin