Ever wonder why long arm kits came out? Well they did because with the shorter arms the rear (and front on TJ's) would move downward and forward hiting the front of the back fender and the back of the front fender with anything over a 4 inch lift. The stock arms and their position create a short arc and the rear wheel (back to the KJ) will contact the door flare at anything over 2.5 inches. The longer arms give a nice long arc for the wheel to travel therefore the motion the wheel appears not to leave the center of the wheel well.
When I lifted mine over 4 inches the rear wheels almost came into contact at ride hit with the door flare. So I adjusted my Rusty's arms out to the point where there is 18.5 inches between the bolt holes. About 2 inches longer than stock if I rememeber. I would recomment that you get adjustale arms for a lifted Liberty. As you move the rear wheels back with the arms, you also off center the spring. In addition, as the lower arms get longer the pinion drops down, rotating the diff forward. This will cause a vibration. So you are correct in saying that a adjustable top arm or boomer rang is needed. To solve my problem at 3.5 inches (almost 4 in the back) you can see in the post under "Lift Experimntation".
http://www.lostjeeps.com/forum/phpBB3/vie ... imentation
Here I took the ball joint out and welded a "johnny joint" to the boomer rang futher back causing the pinion to rotate up.
At 5.5 -6 inches I will be building a new tri-link this weekend to really adjust the pinion. At this height you have no option but to go for a new driveshaft, the stock one vibrates too much. I had one made with a slip yoke from a GM tranny, and a double u-joint (cardone (sp?)) joint. This type of set up you need the pinion at the same angle of the driveshaft. Its still a little off so I get vibration from 0 to 35 MPH, than it stops, or you can't feel it. I need to adjust the upper point of connection, meaning a new tri-link.
With the extreme rotation of the rear axel to adjust the pinion on my KJ I needed to cut the spring pads off the diff. and rotate them to so they are parallel with the upper perches. And there is now about an inch difference in the center line of the upper and lower spring perches which makes the spring look weird. 9th degree (great TJ lift) has solved this problem with upper spring perch relocators. They move the upper perch back and tilt it at the same angle the lower spring perch wines up at when that kit rotates the rear axel (pinion) upward.
Did this help or just confuse things more?
Al