In this context, a "dually" is a vehicle with two wheels on each side of the rear axle. This is a common configuration for heavy-duty work trucks, such as this Rolls Royce.
Though it is normally a luxury brand car, a Craigslist seller in Tampa, Florida owns this Rolls Royce work truck version. It's a custom vehicle with a crew cab and a flat bed. From the Spirit of Ecstasy hood ornament to the trailer hitch, it's an original look. Now you can be chauffeured around town in style.