Why have you pushed the gearbox output cup so far forward of the front wheels axle line?
The universal Joint used in rc cars will cause chatter when run at a angle. Due the of velocity sinusiodaly changing dependant on angle that the joint is bent at.
See the maths here
http://en.wikipedia.org/wiki/Universal_joint
It a potential stress and vibration problem and means in a straight line the wheel velocity is contantly changing.
Also the rear trailing arms are long are you putting track arm
at the bottom of the rear wheel hub to control the side load?
You may have an issue with the hingepin being subjected to a bending moment whinch fatigue the arm or snap the are and the hinged joint.