The angle of the tailshaft doesnt matter, but you want the angle of the gearbox output shaft and the diff pinion to be the same. Angleing the diff pinion down is just like angleing the gearbox up.
They say that ideally you want the diff pinion angle to be 4 degrees lower than the gearbox shaft's angle. This means that underload when the springs windup a little - both angles will be the same. ( realisticly every car is different and springs loadup differently )
Different angles cause the uni's to rotate at different speeds throughout a complete revelution and this causes the vibration.

4WD shops sell angle shims for between the diff and the leaf spring.