Ground faults/loops ALWAYS suck major a$$...
At least it led you to your bearing failure before anything got more serious on the mechanical end...
Troubleshooting ground problems almost always involves eliminating the grounds, one at a time, until you find the one that causes a change... They can be anywhere, and provide maximum hair rip-out power even in the 'simplest' of causes.
My hat is off to you, find it before it does more finding you. That's my best advice. In the case where multiple grounds are at work, this can mean very time consuming troubleshooting, and potentially more damage (like setting dumps on fire) if you don't locate the source of the problem. IMHO, it's better to take the system COMPLETELY offline and look for the problem step by step rather than looking for it 'later'. You'd almost certainly be sorry.
I know this isn't much help, but rest assured I feel your pain, and hopefully you'll ultimately find it, and post the results if nothing else to help a newbie.
I used to work on car electronics, and let me tell you, when a ground goes nuts in a car, it can be one of the most nerve racking experiences. I'm sure that a mill issue is several times that, but believe me when I say I know how irritating it can be.
Good luck, be persistent, and don't give up until you find it, lest you enjoy throwing money away on smoked equipment (as you've already seen...)
Steve