Kevin,
Could be an air leak. Most obvious place would be the carb boots. You might also check the carb diaphragms and the O rings on the butterfly shaft. Air leaks will be more obvious at lower RPMs when the manifold vacuum is highest. One way to check for air leaks is to spray a little starting fluid around the boots and O rings and see if the engine speeds up. I would sync at the lower RPMs and not worry about slight differences at WOT. Paul may be right also. it could be engine mount rubbers. What is your idle speed? Sometimes if you get much below 1800RPM, you will get some vibration. Have the carbs been cleaned lately? if not, you could have differences in fuel air ratios between carbs. This would show up pretty quickly in EGT readings. Worn jets or jet needles could also cause air/fuel imbalances.
This may not be a single problem. could be several small things together causing the problem.
Bill.
