No harm in jetting in some WD-40, but I recommend something better like PB Blaster or Knocks-er-Loose. WD-40 isn't that great a penetrant. When you get the old plugs out, be sure to use anti-seize on the threads of the replacement plugs to make your life easier on future R&Rs.

Check the rotor for a small set-screw. I don't recall whether Isuzu uses this, or not, but it should be simple to check.

Randii