Something tells me you aren't talking about an 8 inch, they are removeable carriers like the 9 inch, the 7 1/4" 6 cyl rear ends have a cover plate at rear of housing, is that what you are talking about? If so, it may just be some gasket material or silicone. If you are talking about a removeable carrier 8", then do you have the axle shafts pulled?-they would have to be out to remove center chunk.
The axles have a retainer to hold the wheel bearings captive. Remove the rear brakes then you'll see the nuts that have to be removed. Remove them then use an impact puller to pull the wheel bearings and axles from the center section. Also you should buy a shop manual.