I had my oil changed today and the guy came back and showed me my differential plug. and it didnt look to good. It had some signs of very small shavings and a bit of what looked like sludge. I asked him to go ahead and service it since I was there. Im not sure but I didnt see any signs of them removing the housing cover.. Is the another way of draining the fluid out and adding new fluid without removing the housing ??

 

yes, if they did it!

They probably used a suction tool! I bought one awhile back and it makes things faster with less mess, they slide a tube down in the fill plug, then suck it out!
the only downfall is that you cant remove all of the fluid or inspect the gears!