I think you need to check when it happens - if it's there when you're putting power down then it's most likely drivetrain, if it's there all the time (even on a trailing throttle) then it's more likely wheel balancing. I know my front prop produced a rhythmic vibration if I booted it at about 60mph or so which disappeared when you took your foot off, which turned out to be a need for new UJs.
Bushes, however, could well show up as both - they may be allowing the axle to move under load thereby causing the vibration!
