I have run into several folks at Murray that are fishing exclusively for Shell Crackers in the past month. I have never fished for them in Murray or Greenwood, only Santee in the spring. Apparently the fall bite is good if you know where to look and how to catch them. I was told they caught them on night crawlers fishing rocky points.
That info is worth exactly what you paid for it, but all I've heard on the subject.

Here is a link to SC Sportsman article which might give you some insight. Lake Murray Shellcracker Fishing • Freshwater Fishing in Sand Hills Area • South Carolina Sportsman, SC