After reading about the main seal leak, I can't help but think the damage may have already been done. My advice would be to do some investigating on the lower end. Even if you find some internal/external balance components have been mix/matched to cause the vibration and correct it, the damage done to the bearings, etc. is unknown at this point. Me, I wouldn't hesitate to yank the motor and tear into the bottom end and get to the root of the problem. Just my .02.