Phalcon,

If you have not done diff before, plugs will be pretty "set" normally and you may well need a breaker bar to loosen them. Most importantly, loosen fill (upper) diff plug first; this way if you cannot free it up, you have not already drained fluid (by removing drain plug first) and gotten yourself into a pickle.

Finally, try to catch all the fluid in your drain pan and do not let it get on your garage floor - the smell will linger in your garage for quite a while if you do - and it is most pungent!