rear axle question

You need to fill the rear up till it starts to run out of the fill plug...

Yes, if filled to the proper level, the diff fluid will travel down each axle tube and lubricate the end bearings. It sounds like you need to drill out that old fill plug and use an EZ out to get it out and reinstall a new plug and a new cover. Fill until it runs out of the fill plug. If you don'tdo this you'll always be guessing about the amount of fluid you've got in there. Just because somebody else (a previous owner) screwed something up doesn't mean you have to live with it forever. Fix it right. You'll be a lot happier in the end. Plus, it's a cheap fix!