I had the same issue on my 1500. It was so low that the helper spring kit actually made it worse and bottomed the bag out so it rode like a rock when aired out. So this is was I did and it should be the same process for the duallies:
I used the upper brackets from the helper spring kit, plated the frame and bolted the upper brackets to it. I got a set of behind the axle lower bag brackets and it work out perfect. I also used a bigger bag too.