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Please allow me to electrocute myself....
Yeeowwch!!
Me being me, I would probably drill a small hole in the floor & run a ground to the nearest cold water line. Note I'm not suggesting you do this, as it is certainly nowhere near code :).
Heck if you're doing that you might as well run the ground back to the box.
Well since you've gone and practically run a new line you might as well run a new line!
You have nice equipment, I suggest you simply run a dedicated circuit. Even paying someone is usually <$100.
Good Luck!! (and if you're a DIYer don't forget the tester - & be careful working in the box!)
Pete