In reference to trucks, means having two rear wheels. Almost always, the phrase "crew cab dually" is used to describe your standard heavy duty farm truck. I try not to be a grammar nitpicker, but trucks of this nature are NOT referred to as pickups. Pickups are single cab, and usually have just one set of wheels in the back. Just a heads up if you're ever in the boonies.

One other interesting thing about duallys (that is my personal preference for the plural form) is that should one of the rear tires ever lose pressure, you can run on just one. There are a host of interesting tire related issues that deal with wear and treads for duallys, but I would be overdoing my expertise in this field to node them as fact.