If you want to lock the diff up, then the best way is with bicycle inner tube repair rubber.
* Go to your local bicycle store.
* Buy one of the inner tube repair patches (flat type).
* Take the ball diff out of the chassis, and dis assemble it.
* Cut the inner tube rubber to the size of the pressure rings (marked 305082 and 305050 in the diagram).
* Remove the smaller ball bearings that sit against the diff shaft.
* Place each inner tube rubber circle between the diff shaft and pressure rings.
* Tighten the lot back up and place the ball diff back in the chassis mount.
This diagram should help you see what I mean...
Now go rip it up with your freshly locked diff.
Works a treat, and will NEVER slip. And not a bodge job like the blu-tack or loctite.
I never had a problem with mine.
Hope that helps.