Quite a few times I've come across front diffs that leaked from gasket between the axle housing and Diff carrier. It's almost always loose nuts. However while out at Cruise Moab on my final trail run @cruiserdan spotted a large oil spot moments after we parked for lunch and sure enough the nuts were quite loose and the gasket was torn. Luckily we were near the end of the trail and while I had a 36 hour drive home I'm part time so not a problem. Dan mentioned that Toyota at some point eliminated the lock washer for a flat one and that was likely the issue. So I went on the hunt for better nuts and found some that I like. They are a JIS serrated flange nut, class 10.9 and zinc plated. Today I tore apart the axle to replace the gasket and install the new nuts. As it turns out I did have lock washers they simple didn't do the job for me. Attached are the before and after pics of the diff along with one of the nuts I used. It takes 10 for an open diff. This is just the front diff and I'll get some for the rear and swap them out as well.