Yes some oil will come out.
And yes you can add some oil when you put it back together.
And its just differential oil.
Check you owner manual for the correct type.

And as for getting it back in there. You might want to put some oil in a plastic baggy and carefully place it back in there. Once you drive it, the baggy will disintegrate, and deposit the oil.

If you donÆt want to add oil just make sure that it is full of oil in the ômainö part of the diff. The oil has to seep past a bearing to get back to where the axel disconnect is but after a couple hours of driving it should get oil back to that area.