Jobs don't come much simpler.
I would think you have a limited slip rear diff, especially since Steve stated you need GL5. Stop by the parts house and look at their gear oils, see if you can find what Steve recommends.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.