OK, I think I understand now. When I had the valve job done I removed the rockers and replaced them after I put the heads back into the truck, if I remember correctly. I probably knocked the plug out of place without knowing it before I tightened down the caps. I will try and replace it by loosening up the caps as suggested (I still have the old plugs, but thanks for the offer.)

89 Montero 3L. Stock.
Rust and Repairs at Equilibrium