I am guessing since you mentioned a 4" RE kit that it is a YJ, so did you shim the rear axle at all? What about a t-case drop?
The kits are pretty easy to install, the only specialty tool you need is a pair of snap ring pliers. However you will need a CV driveshaft after which will run you about $450-600 depending on where you get it, and you may have to cut your spring perches because you need to point the pinion yoke at the t-case yoke
Talk to National, Modern, North Shore, Hoyboyz, they all carry them.