7.4v seems a bit high for the servos, do they ever get hot?
if anything, you should use the 9v for logic (regulated) and 7.4v for servos.
the only reason admin suggested two packs was because of possible "spikes" caused by the servos that might cause resets with the microcontroller.
since then battery technology has improved so much so this is not really needed. the cap also gets rid of this.
if the servos are struggling with the load, there isn't much in the way of software you can do.
if you try just putting rubber bands around the tires, this can help with traction while still not making the load much greater. i would think the servos "jitter" is caused by too much load placed on it, after all its only a micro servo.