You should need a 14mm hex drive to remove/tighten the filler and drain plugs. The drain should be located on the bottom of the diff housing and the fill should be a little over half way up on one of the side of the housing, probably the rear face. With a funnel and a milk jug you should be able to drain the gear oil without making a mess. It should take about 2 quarts of gear oil to refill. Be sure to remove the fill plug before attempting to drain, just in case the fill plug is frozen in place (you need to be able to refill
You should be changing the gear oil every 30 or 50K. If you've never changed it, or have no record of it being changed, now is a great time to do so.
'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.