It is always a good idea to replace the crush sleeve. If I remember correctly from my days at Reliable Truck Parts, the crush sleeve is on the pinion yoke and creates a smooth surfave for the seal. Now, what usually happens is a small groove is worn into that sleeve. This will prevent a new seal from sealing; the old groove will not line up with the new seal and it will have a small, but annoying leak.
Stuff you will need:
Inspection cover gasket
Micrometer to set the backlash
(you should be OK if you use the shims that came from the factory, and you don't change the gears, instead of re-calibrating the backlash)
The above list is for if the pinion bearing is your culprit. Make sure you have another vehicle to drive in case you run into complications, such as bad spider gears or flaked ring and pinion.
Axle bastard knows what he's talking about and will probably find something I forgot to add to this list.