... and when you rotate it (to make it easier on yourself) mark with a paint pen or something, the relationship of the driveshaft to the rear flange as it is before you rotate it. It is balanced to be in that position. Since you have vibration, rotating it may help by shifting the balance thereby making your vibration less/go away. Reason I say mark it is just so you will know how much you are rotating it (just for your reference)...