[Dan Heck]

I'd do just what you have done-only perhaps use some thicker rubber pads to absorb more of the motion. It seems that something's pretty far out of balance if there's that much vibration when the grinder is in use. It might indicate a dangerous situation. Is the wheel chipped or wobbly? It might be a good idea to remove one grinding wheel or brush or whatever you've got at a time and see if the out-of-balance situation moderates. Then maybe trash that wheel for a new one. At least make sure the guards are in place and use your goggles in case something lets loose: I saw a guy on one of those emergency room tv shows a year or two ago with half a grinding wheel sticking out of his forehead....

Dan