Yeah there is an o-ring in there. I replaced it when I rebuilt the rear end.. I also used black RTV on the 3rd member when i installed it into the houseing. No leaks there!
it just looks like its seeping out through the shims, and only on the passenger side...got me confused.
If it is leaking between the pinion support and the case, it has to be the o-ring. It is possible to damage it when it goes into the case. I would double check it and maybe use a little silicon there as well. I can't think of any other reason for a leak there.