While you can just go with long coils like the skyjacker coils, you also need to consider what that much lift does to the rest of the suspension geometry.
You'll need new shocks (without a doubt), and definitely should consider a drop pitman arm, and a track-bar relcation bracket to correct the steering, because you're going to get some nasty bump-steer.
Also, with Skyjacker's 5.5" kit, they relocate the upper control arms, and extend the lower control arms so that the suspension is operating at better angles than the stock geometry would be, once you put that much lift in.
There's a reason why Skyjacker sells it as a kit...