you are in the right track - you can find the vibration by removing the shoe drive belt and other belts one by one until the vibration goes away. If it doesn't it is your main drive shaft either brg down or as stated in the clutch or rh Var Speed. I have seen one main clutch shaft warped on rh side due to a massive injestion of a chunk , just a 1_8" runout caused the vibration.