How do I lower/level rear of 2011 GMC 4X4?

[JAG66]

Need to lower my father in-laws 2011 GMC 4X4 about 1.5" - 2" in the rear to level the truck. Anyone done it and what do you recommend?
I assume longer shackles, but looking at it, the factory shackle bolt heads face the frame and I don't see how I could get the bolts out of the shackles.

Anyone removed the bottom leaf overload spring and how much does it drop?
Anyone remove the 1" thick block that is under the leaf pack?

Thanks, Jeff