[jbsoukup]

i had the same symptoms and could not figure it out for a long time. eventually just by chance i loosened the set screw on the coupling, put it in forward so as to push the shaft in, (at the dock, of course, and i don't remember why i needed to do that) and behold! the vibration was gone. tightened the set screw and it came back?! turns out the coupling was the culprit all along; although i could see no damage and it seemed to fit the shaft fine. i suspect that after removing the coupling ( it was Not easy) it became slightly deformed?

so new coupling and smooth as silk.

also, by the way, i felt real weird about relying on just a couple set screws keeping my shaft secure in reverse so i drilled a shallow detent in the shaft for the set screw to set into.