Modifying QX56 Suspension

[logicool1]

I have a 2012 QX56. It looks like it's beginning to sag in the back. I no longer hear the compressor kick on, so not sure if it's the compressor or the shocks. I have seen in other threads mentions of modifying the suspension. Has anyone done this? How does the truck drive after modifying the suspension? Approximately how much does this cost?

Thanks in Advance!