Check the first post in JK Tech "Newbie's Guide to Lifting Your JK"; it explains the purpose of just about everything.
JK or JKU?
I just had the RK 1.5 X-Factor installed on my JKRX and had to do some adjusting to get caster into spec. RK says that the lengths they provide in the kit install instructions are a guide but each Jeep is different so you'll need to get an alignment to check caster, etc.
Not just an X, a xXx, she's gonna get dirty!
'14 JKRX - Granite, 6-speed manual, soft-top, 430N, side air bags, Auto A/C. Mods: JWS8700Evo, Maximus-3 Stinger, Granite painted interior trim, 13" Antenna X, Rampage Locking Hood Latches, MC Grille Guard, Gibson Exhaust.