Shocks and struts

I think my shocks and struts are messed up, cause i hear a sound on the passenger side rear everytime it hits a bump. Anyway, i was wondering, there are 4 shocks and 2 struts....on the rear? of my mustang....or how is it arranged. What needs to be replaced? Is it ok to replace just the shocks? Or just the struts? whichevers making the noise? thanks for your help
 
Two front struts, two rear shocks, and two horisontally mounted rear quadra shocks. You didn't mention if you still had the original stuff in there? If so, it's probably all pretty evenly toasted by now, 1989 wasn't exactly yesterday. Just replace it all. However, your noise could be coming from any worn out oem rubber too. Chasing down suspension noises in these cars can become a long process of elimination.