Unless someone for some reason changed your stock setup to Shocks sorry to say you have Struts in the front of your 91 not shocks.
The 1979-2004 Mustang front suspension consists of modified MacPherson struts with lower control arms (A-arms). The term modified refers to the fact that the coil springs are inboard of the struts. Normally, MacPherson suspensions have coil springs that surround the struts. This suspension is compact, cheap to build and easy to service, but it is often criticized because (a) its geometry produces camber changes during vertical suspension movement and body roll that adversely affect handling and (b) the strut towers prevent designers from lowering the car's profile. The performance of the front suspension can be improved by installing stiffer springs and struts and by replacing rubber bushings in several locations with less compliant urethane bushings.
The easiest way to tell the difference is your strut will have a bracket on the side with holes so that it can be fastened to the spindle, the shock does not have an bracket on the side .
Shock on the left............ Strut on the right