[Overdriven]

Get some silly putty and put it in the center diff. That'll lock it and is also reversible if you don't like it. Grease doesn't cut it in 1/8 scale though, silicone fluid only.

The Mayhem (thats what your's is right?) I just built for my nephew's xmas present didn't like 10k much, still unloaded to the front alot balloning the front tires bad. Ended up mixing equal (or pretty close to equal) parts 10k and 30k and its perfect now.