Glad to hear you found the problem with the center diff.
As for the rear diff, make sure you have a gasket (I think its plastic, but not sure) on the inside of the spur gear. I accidentally threw it away a while back because I had no idea what it was. Low and behold I needed it... so digging through the trash I went to find it. I did find it by the way.
I still have the gasket in place (blue coulered). I think I'm going to order a new one.
Also there is this little grub screw in the diff. I wonder if the oil is leaking through it. Any idea what to put on it jus in case. I don't thins thread lock will work with plastic.