I think what you're reffering to as the track bar is actually the sway bar and yes you can remove it with only ill effect is that is the body may roll a little more around corners. the advantage is that you will get a little more articulation when you're wheelin. I dont believe XJ's have a rear trackbar if they have leaf springs. ZJ's have a track bar because they have coil springs and if that was removed it would definitly be BAD THING. The sway bar is mounted to the top of the leaf springs and mounts to the frame on both sides on pivot points. Track bars are mounted one side to the frame and the other to the opposite side of the axle and is designed to keep the axle aligned with the front wheels.
someone feel free to correct me if I am wrong about XJ's not having rear track bars, I am not real familiar with the newer ones.