you may be able to get away without them, but most likely you should expect flickering and/or only 1 light to work at a time. depending on the condition of your stock wiring.
it is desireable to add a new harness, and downright mandatory (from my experience, all the jeeps in my stable run HID's everywhere possible) to ground back to the negative battery terminal.
i cant recommend enough that you build your own, via go-jeeps website, it's super easy, and if you dont know anything about 12v electrical, building the harness will be your primer!
i also prefer low-only kit's, many hi/lo's use regular halogen bulbs for the hi. i prefer to use my rallye 4000 HID's for that.