ALWAYS ever loosen the filler plug first. If you dont manage it, you have still oil in the diff. If you drain the diff and cannot loosen the filler plug you´re f*ckd
To loosen the plug take a saw and shorten an allen key then put a wrench with a rtachet ring over it and get out the plug. There is no space for standard ratchets. To fill up you might use a tube and a syringe for there is no space for other things....