I checked now some enties in the forum and it seems that other people had the same problem and the only way to solve it is to tighten the diff.
But don't you think I could get a problem if it is tightend that much?
I want to go with this guide:
http://www.rctech.net/forum/electric...er-clutch.html
It says that I should hold both diff exits and then tighten it so much, that I can not move the gear anymore...
That's exactly what I did when I built it, but then I faced the problem with the driveshaft and t nut...