Check the grounds as suggested. Also check the hi beam switch on top of the column. Do you hear or feel a click when engaging the hi beams? There is a rod that runs down from the MF lever to the hi beam switch. If it has fallen out or become loose it will cause no function.
I just recently had a similar problem...my left hi worked, and my right lo worked, but not my right hi or my left lo...just so happened that they coincidentally went at the same time! Two new headlight fixtures and I was back to seeing when I drive at night. As everyone else has said, grounds are a pretty common issue, so I went and cleaned all mine up. Not that hard of a task on the YJ's.
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