I dont know if you have mobility issues, but what about getting steps? If you lower these trucks you lose clearance, suspension articulation, as well as either ride comfort or payload capacity. If none of this is a concern, I wish you luck. My google foo has failed at finding anything useful. I can find stuff for the 1500s but nothing for 2500/3500 past 2010.
You might just go to a suspension shop and ask them to do something custom. I'm sure it wouldn't be too terribly hard for them to fabricate some drop brackets or, at the very least, get some drop springs that will work with your setup.