Replace the slab, but only after properly preparing the base for it. An improperly prepared base is the most likely culprit for the subsidance you've experiened. Your local building office will be able to tell you how deep you'll need to dig and how much gravel you'll have to tamp down before you can create the form and pour.

Thanks for your input. We have been leaning that direction. I remember some discussion about it not being prepared properly in the first place.