I only have an unproven theory that may help you on your way. Massachusetts was a battle zone for much of the Revolutionary War, and it wouldn't surprize me if some left as refugees from the area, and headed to South Carolina. South Carolina was also a battle ground, but not as heavy as what Massachusetts faced. South Carolina became important towards the end of the war, and possibly you had New Englanders fighting in South Carolina who decided to stay? One of the famous New Englanders fighting in South Carolina was General Nathaniel Green.

As for research, I can offer my suggestions based on my experience about where to look if you've only checked out ancestry.com to date. pm me if that is the case. There are other records besides Census records.