Going ever so slightly left-field.... I'd get a Jubilee clip and attach it to the shaft suspected of being the cause. Not sure if you've seen this done - it's in pairs when for real and the heavy parts of the clip (the screws) are arranged such as to balance the shaft by trial and error. However, if you try one and are very very luck (not going to happen!) you rebalance the shaft. More likely you make it worse but it is still the same vibration. Or, you introduce another vibration which suggests the problematic one isn't with the shaft. If you try this, be prepared to move the Jubilee clip around length wise ie, each end and mid position (any imbalance on the shaft could be anywhere along its length).
Not a solution (though possibly could be), more a diagnostic effort.