A little difficult to diagnose from your description. Did it occur all at once? You say sagging, we say how much? Have you checked for broken springs or mounts? What are your measurements to fenders on all 4 corners? Shocks do not hold a car up so you can eliminate them unless you have coil assist or air shocks. Torsion bars affect the opposite corner, but in your case if the left front weakened or "relaxed" it would raise the right rear, making it appear that the left rear was sagging. Don't know how much this helps you, but more info would help with suggestions from the forum.
