N.J. Stat. Ann. § 40:41A-76

The office of board president shall be deemed vacant if: the incumbent moves his residence from the county; or he is by death, physical or mental illness or other casualty unable to continue to serve as board president. Any vacancy in the office of board president shall be filled by the board of freeholders, which shall appoint one of their number to serve as board president for the remainder of the unexpired term. During the temporary absence or temporary disability of the board president the vice president shall serve as acting president.