Have a new 2013 single rear wheel cab/chassis but the custom bed we had made would be better suited on dual rear wheels. What is the most cost effective way of converting this? Is there a significant difference in the rear axle on a single rear wheel opposed to a dual rear wheel? Is it just a lug change?

 

First off you will NOT change the GVW rating on your present axle or truck by converting to DRW.

You WILL experiance a bit more stable ride however.

You might try Welcome - Arrowcraft :: Helping You Handle the Road for over 30 years! they seem to have quality adaptors.

 

You should have thought about that before you spent $45000 on a truck. Maybe sell it and buy a new one that is the correct model.

 

Sorry to be so harsh, but you should have done more research before pulling the trigger on this project.
The entire rear axle housing is different. There's no way to increase the certified ratings. If you need more towing/hauling capacity, you need to buy a different truck. If you just want the dually look than you can do the swap but it hardly seems worth it.