You can get plastic pellets (on eBay) that melt in boiling water and then becomes pliable for a few minutes, push some of that in and it's removable afterwards for grip racing, then put it back in for drifting

All planetry diffs can be locked this way

Ball diffs are a different matter, I locked mine by removing the balls in the spur gear and tightening up the diff so it didn't slip - works too

Kev