Usually it is the track bar that makes contact with the diff cover. Commonly caused by aftermarket front diff covers, aftermarket track bars without the correct bend, lack of proper bump stops to keep the parts from making contact, or a combination of all those things.
No, it is absolutely not ok to run without a track bar, and it is not ok to leave it that way. One good bump too many while you are traveling down the road could break the track bar and the Jeep will lose control.
Is it stock? Can you post some more pics? Include pics of the bumpstops inside the springs.