My 05 GMC is like this, its a special edition appearance package, basically built on the VHO platform sans the LQ9. I would venture to guess that yours is the Chevy equivalent and built on the SS platform.
Having said that here are our options as I understand it
1. Spindles up front, hangers / shackles out back
2. Keys up front and shackles out back (which will net you far less drop than option 1, I think usually a max of 1/3)
So if you want 2/4, 3/5, 4/6 etc then its spindles and hangers / shackles.
Just what I have found as I was confused by this as well.