Here is a classic situation where it is important to do it properly. You need to get an angle finder, available in lots of tool stores, and make the pinion angle match the tcase angle. Then you won't have vibes. But since you are going to use a shackle, I can guarantee you will have vibes to start with because you will have changed the pinion angle. Here is a diagram. One of these days I will post a "Driveline 101" on how to set it up right every time.