Let me preface this by saying: I am NOT more knowledgable that Copper, but let me atleast empart some of the knowledge I've obtained on this topic as a result of recent research. Copper and I have been chatting about this in another thread.
Lift causes an offset in the axel, as a result of the angle from the frame mount to the axel mount of the trackbar. Greater height requires greater length, and pulls the axel toward the driver's side, causing problems in flex, wobble, vibes etc.
HOWEVER, you probably don't need one, as I've observed that the 3.5" RE kits don't include one, but the 4.5" do. Of course, every vehicle is different, and the quirks of your XJ may differ than others. I say, install the kit, and if you can visually see your axel justified to one side, then you get the adjustable trackbar.
ALSO, another observation while perusing the RE website. It looks as though the trackbar included with the 4.5" kit uses a heim joint anyway. Those are prone to wear. When you do upgrade, get a trackbar with a poly bushing end.
EDIT: I also wanted to add that the Rusty's heavy duty trackbar is less than the RE track bars with the heim joint. (the RE bar doesn't include a new frame mount too)