Check if metal junction box is "charged" from bad wiring
I am addressing your question first. You can't use a voltmeter to check the box by putting one of your probes to one side of the box and one on the other side. You need to kill all power to the electrical circuits in the box and using you continuity tester to "ring out" which hot wire is going to ground at the box.

That being said, replacing the metal box with a plastic does not fix the problem of a shorting circuit. It simply removes point of grounding which allows the short to trip your overcurrent device (the breaker). This means anyone who comes into contact with the box and is grounded will be subject to electrical shock or a burn hazard. So the box is actually telling you that you have a problem with a circuit inside the box and it needs to be repaired ASAP.

So your wiring is in need of repair and you should address that problem immediately. As @SDsolar has said if you are not skilled enough to make the testing and repair. You need to get a professional to do it immediately.