If all you want to do is change the rear differential oil you could leave the drain plug where it is and suck the old oil out. The tool for doing this used to be some where in the $13-$20 range. See picture.
You can get one at the local parts store.
Another option is to leave the plug in and loosen or remove the differential cover, drain, and reinstall it with a gasket or sealant.
Go to the junk yard and get another differential cover with a decent plug (remove the plug at the junk yard to be sure it will come out).
Otherwise you will have to do what ever is necessary to get some sort of Eze-out out into the plug to turn it out. Usually requires the proper size drill, Eze-out and sometime a tap to restore the threads.
They may have a newer type of EZE-OUT now for stripped internal allen/hex type slots that you will not have to do any drilling to use but I have not seen them or have any experience with them.
Another messy option would be to drill, tap, and thread the existing plug for a smaller american pipe plug. When the drill enters the differential the draining oil will was out the chips. When you use the tap put thick grease on in the tap flutes (grooves) and the grease will trap the cuttings as you thread the tap in.
Motor Drain Oil Fluid Suction Siphon Pump
