A TJ comes with a transfer case that has what is known as a "slip yoke". This is where all the movement happens as your axle moves up and down. When you lift your Jeep high enough (3" or more), the angle of your rear drive shaft will be steep enough to cause it to vibrate. Not only is this annoying but, if you leave it unchecked too long, you will do damage to your u-joints and can potentially break your transfer case.
A slip yoke eliminator or, SYE will replace the tail housing of your transfer case with one that has a fixed yoke. This will in turn allow you to install a standard CV drive shaft where the slip occurs on it. However, to install it, you will need to set your pinion angle so that it is in line with your drive shaft and this is where adjustable rear upper control arms are needed.
You can get by all this by dropping your transfer case skid plate but, this is really a terrible idea and one that will rob you of all the ground clearance at your belly that you just paid so much to get higher off the ground.
Here is a link to a write-up I did way back when. It will show you what all is involved:
http://www.4x4xplor.com/SYE-CV.html
