The plug your seeing is for filling. You drain it by loosening and prying open the bottom of the cover. Once drained, remover the cover, clean out the gunk on the bottom. Inspect or replace the gasket , then reinstall the cover.
Fill to the bottom of the fill hole. I use 90 weight multi-purpose gear lubricant. If you have a posi rear I believe you will also need some type of additive.
