mugen mbx5's diff's had the same problem and this was the fix i read on these forums. assuming your not overfilling your diffs. the likely culprit is a warped diff housing. i found 2 things that worked to fix it. 2 gaskets will seal it, but you end up increasing the length and its a lazy fix. the other is get some sandpaper, around 600 or so. lay it on a table and lay the diff cup on it, up side down. while keeping the cup flat on the paper, rub in a round motion. peek at the seating surface after a couple revolutions and if you have a consistent surface on the lip of the cup your done. if not, repeat till you do.
would you say i have a plethora of pinata's?