depending on how low you want to go there are few options. KP components makes a bolt on rear 4-link, and Airide Technologies make a kit I believe also. The front is pretty simple, cups and a bag, and a set of dropped spindles.
Kelderman makes a kit for Denalis and Escalades (to lower) and I believe they said the front will work on the pickups. The rear is pretty simple adaptable.