Try Ceramic Balancing Beads, I was having balancing issues when I started doing Phat Tire conversions on Speedys and Americas. 200 mm rears and 140 mm fronts, works with Mags and Spoked Wheels. You need to get special stem valves for inner tubes. The best benefit is your tires stay balanced as your tire profile changes due to wearing in. Obviously, this will not fix drive train issues. They stress in Industrial Vibration Analysis training programs that all most all machines that spin have a RPM range where the vibration axis jumps up to an undesirable level, our problem lies with that they are referring to startup to attain a specific operational RPM, whereas we are striving for smoothness at variable RPMs. As far as ascertaining where the offending vibrations exactly come from, they have stick on, memorized, self powered sensors, readable by a laptop with the appropriate software installed. As suspension/alignment tuning shops do exist, they should offer a service such as this as well, I think they would make money and if I had a problem such as yours, they would be getting some of mine.
