I don't know if you can go to test and tune but try shifting into neutral right when it starts see if it goes away...if it does its trans forward if not its in the drivetrain etc. come up with a way to pinpoint. after that listen feel for frequency if the vibration is with the speed of the wheels etc.
Thanks but I would have to back off the throttle to shift from 3rd all the way to neutral, and simply backing off the throttle causes the vibration to stop so there would be no need to shift to neutral at that point.