I got your pm and sorry for the late response.
I just don't come on here like I used to.
You can use a TA04 gear diff if you can find it?
Another option is to use the Traxxas slipper friction pegs. I remember seeing some guys use the friction pegs as well as some sand paper cut to the correct size shape of the ball diff rings and locking it as tight as possible.
The TA04 gear diff is the best way to get a locked center diff.