[MEvang]

As you see here there are two oil seals. The one it the top illustration is the one that will leak causing the mess. The oil seal on the bottom illustration is there more to keep water and dirt out. Both are important, replace them both. Also replace the O-ring which seals the bearing retainer to the diff housing.
The bearing, which will need replaced is held on with the retainer. Both are friction pressed on and not reusable. The snap ring may be reusable but since it's been off once you may want to go a head and replace it, why take a chance. The gasket the goes on the Oil Deflector will need replaced too.
If you have ABS, get a new speed sensor rotor. Sometimes these can be saved but many times they are damaged when removed. If you see any damage to the teeth or and possible distortion, replace it. This can affect your ABS operation. You can always return it is you don't need it.
Basically all parts you see with a asterisk beside them will need replaced. The one part you can eliminate is the gasket between the drum and axle. This tends to cause more problems then it's worth. Just make sure there is no old gasket material remaining on the axle or drum and use a bit of silicon grease to prevent rust build up.
Mike