The Duratrax overdrive diff is the weakest piece of crap I have ever dealt with. Overall, I think the whole truck is a piece of crap.
I just got through rebuilding one for a friend because his diff went belly up and I came to the above conclusion.
I looked all over for a metal case hop-up to replace the plastic but couldn't find one. There is one pictured on tower hobbies but it's misleading. The photo shows a standard diff with metal case but what you get is some kind of center diff instead that doesn't work.