public void setRollInfluence(float rollInfluence) {
        this.rollInfluence = rollInfluence;
        applyInfo();
    }
reduces the rolling torque applied from the wheels that cause the vehicle to roll over.
This is a bit of a hack, but it's quite effective. 0.0 = no roll, 1.0 = physical behaviour.
If m_frictionSlip is too high, you'll need to reduce this to stop the vehicle rolling over.
You should also try lowering the vehicle's centre of mass
@param rollInfluence the rollInfluence to set
jme3-jbullet/src/main/java/com/jme3/bullet/objects/VehicleWheel.java