twist it up and down to break the seal
there should only be a small o-ring inside to stop the engine oil from escaping, other than that, it's just going to be a case of persevering

are you sure you aren't treating it with kid gloves because you don't want to damage it?

i can't remember as it's been a while, but you should be able to see the inside of the diff from within the sump, a plastic mallet might be ll that's needed to crack the seal

otherwise, if you need to get it off in a hurry, cut the sump away and then just crack the outer cover where it goes around the diff to release the suction

of course, this is always a problem if peole don't use the correct o-ring and just whack a load of fords sump sealer into the gap, it sets like rock and you need to chisel it off