Buy another truck, its not worth the hassle nor the liability. You have to remember the dually has bigger bigger tires, stronger suspension, and bigger brakes front and back. Just doing the rear axle does not bring you up to a dually weight, and if something were to happen with an accident you would probably get sued like crazy.
Why a dually, water is not light, and trucks are not light either. My 2005 Dodge 3500 Dually Quad w/long box has a curb weight of 6900 lbs. Add in the water tank, burner, engine + pump, fuel tanks, hose, and some chems, it adds up quick. Fill the tank up with water, fuel tanks full and the truck runs at 11,400 lbs of the 12,200 lbs total weight allowance. You want to lower your curb weight drop the quad cab, shrink the box, and lose the 4wd, those 3 add a lot of weight to your truck. I needed all three which meant I needed a higher payload hence the dually.
The other big thing is that the police do commercial checkstops a lot out here with scales and pull people off the road for being over weight. I figure piece of mind when it comes to them and also piece of mind that if a kid jumps in front of my truck I can stop as quickly as the truck was intended to do.