Like plaidfish said these diffs are designed to hold oil unless you don't have it together right. This is the first time I'm hearing of this problem but when you close the diff there really isn't that much room inside them just whatever space is imbetween the gears. When you take it apart it should be about half full of oil or maybe a little less.

If you use 1000 weight grease and 1000 weight oil I really don't see why the grease wouldn't find its way out either. Grease does tend to stick to whatever it contacts but whatever you have in there does get turned up quite a bit. If you just pack your diff with grease other than weights that are close to the oil your running now chances are its just going to be way to heavy and pretty much lock the diff.