I would replace the dust seal at the end of the differential tube. I would also use the yellow oring, (the bearing should have this) just to keep the hub cleaner inside and keep water out of the 4wd hub assembly. Even with mechanical hubs!

Be prepared to remove the studs in the old bearings. They are a to remove. Double nut the stud and hold the old bearing in a vice and you can get them out. Don't forget the penetrating oil. I used a wire brush on my drill press to clean out the threads before installation. When you install the studs in the new bearing be sure to bottom out the stud to tighten it.