Looks like I'll be rebuilding my first differential soon. Today while taking my wife to lunch I noticed a loud rumble from 20-25mph. Hopefully it's just a bearing, anyone have any knowledge about this issue?

How hard is this to do? I have access to a lift and almost every tool under the sun I just need to know what I'm dealing with. This issue just started, I'm the third owner so I'm not sure when the differential oil was changed. If ever.