You should put the back end of the car on jack stands and remove the wheel to really see what's going on. Before you do that though you could remove the back seat back/corner panel to see the top of the strut. Have an assistant push the corner of the car down and up and watch to see if you can see something moving that shouldn't be (or something not moving that should be). Compare it to the other side.
Replacing struts on a 91 is not a bad job. Only basic wrenches and sockets and a set of jack stands are needed. Your biggest challenge will be removing the large, rusted-in thru bolts on the bottom bracket.
You should do the rears as a pair. You don't have to open up the brake line. the brake line bracket just bolts to the strut. You'll see.
You should get a Haynes manual but enough information for this job, including torque specs is available at;
http://www.autozone.com/servlet/UiBr...3d8016bd55.jsp