I had this one time also. The problem is that i filled to much oil in! I found it out by running in the diff direct after the build. After this i cleaned it and no more leaking.
If you put in to much oil then it will be leaking out of the orings by the outdrives during the run/heating up. So it regulates the oil levle itself ;-)
Tip: When i finish building a gd2 i put it into the car and run it in.
Car is on the car stand and i hold alternately the left/right rear tire with my hands and hit the throttle constantly on the transmitter.
I do this ~2 minutes , then the diff will reach 25-35 degree.
If you put to much oil into diff it will regulate itselfe during this "run in" method.
In case it dont stop leaking please check the orings on the outdrives. I also saw some who damaged it during the build because the put the orings first into diff case. But this is wrong.
I put o-rings on shafts first and then insert the shaft with bearing and o-ring into diff case.
I also installed and tested the updated FFG 1.1
Super smooth and nice function with the new parts. Feels more free and "softer" now.
It also prevent the problem with the "tweaked/blocked" FFG after a hard crash. Also its no more dangerous to run without the diffcovers like recommended with the first version of FFG.
Best Regards Max