I think the problem may be amplified if there is too much play in the universal joints ie the pins going through the axles should not have any excess movement in the holes. If so take them apart and try rotatinng the pins to the other two holes so they fit better. Make sure to use the ant-wear grease. I know this grease does tend to attract dirt but it does the trick. Last, I think the stock kit using dogbones were not designed for a spool. Your better off switching to the universals. Plus if your using the steel outdrives, switch to the plastic with the aluminum cover, this too will help eliminate the chatter!
Just my 2$