10 milimeter, i just changed my differential fluids a few minutes ago. that's odd that you mentioned about the gasket because when my brother blew the back plate off(is that what your talking about?), i didn't see any gasket, just silicone. looks like you will have to drop it to access all the bolts on the plate. the diff is not light so don't expect you'll be able to toss it around.
gl,