Driving on the beach is illegal anywhere in SC. Fort Fisher, NC is the closest spot where it is legal. You'll find SC to be the least friendly state in the US when it comes to public OHV access. There are a couple of national forest areas that allow ATV's, but even those forbid Jeeps and other 4WD's. There are two private offroad parks in the state. Carolina Backwoods is close to Myrtle Beach, but is pretty much just mud and sand. Gulches is in the upstate and is a lot of fun.