I have been running the Rubicon Epress 2-1/2 for a couple of years now. It was a complete kit: Springs, new bushings, t-case drop, rear axle shims, front sway bar relo bracket, brake line extensions, front sway bar disconnects, and shocks.
Kits call for removal of the rear track bar. The front track bar needs a relo bracket due to the geometry change. I chose to install ss brake line extensions and not use RE's rigid extensions (my preference). I have also since changed to JKS disconnects for the front sway bar (my preference). The one other thing I found I had to do was to install extended bump stops. In a rear flexing situation I was rubbing the rear fender flares with my 31's.
There are those that run 4" lifts without the SYE and new drivelines. My personal belief is that you're asking for trouble when you least can afford it (on the trail). No problem in a daily driver mall crawler but get to flexing and you could have some major issues.
my 2 cents.