Doin good gtmud!
From readin your posts, Sounds like your in the Jeep business and doin good too.
On this little problem, the left plug might be higher and out of the "muk" in the water jacket but you might consider replaceing both of them. While you have the plugs out, run a rod or something into the jacket and loosen any sediment in the bottom and flush it out.
You might find that some of the side plugs are "weaker" too, so maybe this is the time to do the others while you are at it.
I use the teflon pipe thread sealer on the circumference of the plug after cleaning the hole with emery cloth. There are special drivers available but I've used sockets that fit LOOSELY inside the plug. If they fit too tight the plug collapses and grabbs the socket.
JD