I'm dealing with a similar situation. I just rebuilt one of my 3.0's and the rear cam plugs are slowly weeping oil. Going to pull the upper intake and valve covers to replace them. There seemsto be sufficient space to pop some new ones in w/out pulling the heads. I have read info on the original manufactured plugs being to "skinny" and now there are aftermarket plugs that have more depth, but I have yet to find.......

1990 Montero Base Model LWB
1991 Montero LS
1991 Montero LS
1998 Montero Base Model