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.
-via Jalopnik
The advantage of dually wheels is that it's less likely to blow out a tire from a heavy load when 4 tires are sharing the weight instead of 2. That's why the dually wheels are always on the rear axle (under the pickup's bed) and never on the front axle. And why it's always on heavy-duty pickups and never on light-duty pickups.