Bad shock or bad leaf spring

66 dart, L6 270 4 door. I notice that the rear end is not sitting level. The left side is lower. I checked distance from the lowest part of the rear fender lip and it is about 1.5 inches lower on the left rear side. When I get under there and push up, I notice very little resistance in either direction with the shock.

How do I know if the shock is bad, or if the leaf spring is losing some tension?.

The shocks look pretty old, but not sure if they are responsible for keeping the car level, or just keeping the ride smooth.

thoughts? go easy on me, I am NewB and get most of my information from YouTube.