Brian,
Instead of measuring voltage (which tells you almost nothing without knowing a lot of factors you have no way of knowing), measure current instead.
Use your clamp on ammeter (get one if you do not have one) and measure the current in the grounding conductor. That way you will know precisely what you are measuring, and have a better handle on what that measurement actually means.
Measuring 1 volt does not tell you anything because you do not know the resistance, only that there is 'some current flow because there is some resistance'.
Measuring 1 amp tells you that there is 1 amp flowing through the conductor which *is intended* to have no current flowing through it.
If you are just measuring voltage and do not have, and do not want to get, a clamp on ammeter, then I would stop measuring the voltage, because it is telling you so little, and is telling you nothing that you can determine anything else from, or determine what it means or how bad it is.
So ...
... use a clamp on ammeter (get one if you do not already have one).