You would need to change your driveshafts, control arms,front and rear track bars, shocks, front and rear sway bar links, and something else that I might be forgetting. Oh yeah, while you're at it, go ahead and swap out the springs for 6" springs, and sell the spacers.
Basically, if you want to lift 6", you need a 6" kit. A 4" kit will work only for 4" of lift, not 6". If you want a 6" kit, you need to be prepared to shell out between $2500 and $3000, and that's before you get wheels and tires. And if you fit the maximum size tire under a 6" lift, you will not want, you will need to regear, and if you have the D35 in the back, you'll need to junk that and swap in at least an 8.8.
Now, I don't mean to come off as a jerk or anything, it's just that we get this exact question or some slight variation of it nearly every day. I understand that it's easy to think big and get in over your head and budget very quickly. It's good to start off with a smaller lift like the IRO front to rear swap, or the rough country kit you're considering. I also highly recommend installing the lift yourself. It isn't that difficult, and you will learn tons about your Jeep's suspension and how everything works, which will in turn help you understand why you need all the crap I mentioned for an extra two inches of lift.
2003 WJ in Jeep Green Metallic
, 4.0/242/4.10's. 4" Teraflex springs w/ RE monotube shocks and IRO short arms, cheap intake w/ amsoil filter, 2.5" hi flow cat and back exhaust, lots of lights and other misc crap.
Check out my build thread here: Life lessons and budget build