It's all been said; to adjust your XJ's pinion angle so that the u-join-to-drive shaft angle at the pinion equals 0* (correct angle when using double Cardin u-joint at the transfer case--which we are all assuming your NP242 transfer case has??), you will need longer lower, or shorter upper control arms. Adjustable control arms make life easier, but are not essential. Remember, there is an ideal length, considering the amount of lift, necessary to place the axle in the correct place, relative to the wheel well, and to achieve necessary caster; incorrect positioning of the axle will lead to other problems.
So, yes, you could buy adjustable arms and crank in any combination of pinion/caster angles. You could possibly find the combination of pinion/caster that minimises vibration and gives "acceptable" road manners (livable wandering, erratic steering). Certainly, installing adjustable arms will not cause any problems; it's how you use them that may cause problems.
Lifting any vehickle leads to compromises. It depends on what you are willing to live with. If you want to lift your XJ 4.5-6inches, AND expect good road behavior, you'll need to spend time researching the effect of each change, deciding on what mods are required to re-engineer the whole drive line to give the results you desire.
Most guys lift their Jeeps with out huge expectations of "stock" behavior. So such comments as "I've got 6inches of lift, running stock arms, and am not experiencing problems" can be had. You may not experience the same results, because your expectations are different.
When I had a stock D30, high pinion axle, and 6 inches of lift, I noticed some front shaft related vibration above ~30MPH; not earth shattering, but uncomfortable for me to accept--I'm some what of a perfectionist. My caster was at 6* at that point--I had never fooled with it, but I did have longer, fixed-length, after-market U and LCA arms. Obviously, my u-joint angles were off, but I just lived with it, because I had a disconnect front axle--so the front shaft did not normally turn while on the high-way.
If you can't live with the vibration, adjust it out with adjustable control arms. If the correct drive shaft angle results in LESS than 4.5-5* of caster, you may be able to adjust that out with adjustable ball joints, as suggest above. My experience with adjustable ball joints, and large changes, like 1.5-2.0*, is you may create clearance issues between steering components.
In my opinion, the only way to have your cake and it too, with a smile:cheers: is to install a HP front diff, if you don't already have one, rotate the "C's" so that the pinion-to drive-shaft angle sets at 0*, and caster sets at 6-7*. Insure the axle is centered under the front-end, using adjustable arms. AND, if the lift is greater than 5 or so inches, install control-arm drop-brackets, or install mid-length or long arms.
But, before doing all of that, insure your front shaft u-joints are in good shape, not binding at all, and that you do have a double-Cardin style front drive shaft. Also insure it's the right length; IE:, the slip joint is not excessively extended.
