- worn/damaged yoke (sealing area)
- improper pinion bearing preload (the pinion shaft would wobble)
- unbalanced driveshaft (but then you'd feel it so this one would be fairly obvious)
EDIT: that picture is not very clear but it looks to me that the seal is pushed too far in so add this to the list (should be flush wit the housing or very little under), could be your problem.