I found this on the project jk website
Q: What is Caster? How much do I need?
A: Caster is the angle upon which your front axle sits at in order to help keep your Jeep driving straight. Positive caster, which is what you want, will cause your axle end forgings or, axle C's to sit in a way that the top of them will be angled ever so slightly back. Too little or even negative caster angle can cause your Jeep to wander and feel 'flighty' or 'darty' as I've heard some people call it. From the factory, your JK will have +4.2° of caster. If you lift your Jeep and install larger tires, you will want to increase this amount just a bit to help compensate for the modifications. While more positive caster will make your Jeep handle better, too much can cause your drive shaft to vibrate and this is especially true if you are running an aftermarket drive shaft with a higher gear ratio. Ideally, you'll want to keep your caster as close to factory as possible.