Ultimate upgrade for the 9398 4x4 crawler with pendulum steering (no servo needed). I also made a pendulum steering file in the rebrickable link you can download and have a closer look
LEGO 9398 Technic 4X4 Crawler RC Truck Remote Control Off Road Rock Four Wheel Drive on 100outlets.com
Amazon.com: LEGO Technic 9398 Rock Crawler : Lego: Toys & Games