In the late 1800's, explorers had reached the nearby river and needed to cross it, so what they did (which was common practise in those days), was to find a level, easily accessible crossing place where the water was shallow enough to permit crossing to the opposite side. This is known as a 'drift' or a 'ford'. Because this drift contained a lot of rocks, the crossing (and the nearby settlement) became known as Rocky Drift.
Over the years, the name has become distorted to 'Rocky's Drift' is if Rocky was some imaginary hero that lived in the area. A lot of the locals, including the roads department don't know the difference. At least Google have it correct in their street maps and Google Earth.