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The differential will need to be removed before the cover can come off.
Assuming the leak is from the gasket between the cover and the carrier case: The top of the differential cover attaches directly to the rear cross member and can only be unbolted from above. I suppose you could detach the spring from the bottom of the cover and then try to reach around and unbolt the cover from the XMember while in place, but it would be quite a chore.
A leak from anywhere else means you have to take the differential out. If it's the side yokes, they need to be removed from thier channels to replace the seals...same for the companion flange at the nose.