Greetings from a fellow Louisianaian....
Change them all...long as your getting down in it might as well know they are all fresh....
The ohmmeter tests are not very good at establishing true condition of the plugs...
Best way is to bench test them so you can see if they light and how quick....
Ohmmeters just do not push enough current to evaluate condition....
The meter will just tell you if they have continuity or if they are open....
I just use extra heavy jumper clips and a fresh 12 volt car battery or high amp charger....
You can check them one at a time if they are parallel type or rig up the series connector bars to test the old series style plugs on the bench....

(When you see them glow...You know)..

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)