A few thought, can you duplicate the vibration in neutral? if so then yes could be pulleys, get the car rolling shift into neutral and coast is the vibration still there? Basically eliminate things and situations. The four speed could have bad bearings is the vibration in all gears etc. Pu the rear up on a lift or stands run through the gears with the rear hanging,is it a bent wheel etc. its alot of work but if you systematically run through a process of eliminaition, you should get down to if the trans has to come out and apart or not. Keep in mind everythin tgthat rotates needs to be balanced, then there are worn universals,and bearings and bushings etc.
