I started fresh and after going back over Jeremy Blum's book again for some ideas, and following Daniel toll's advice, I started with parts that do work.
And I also switched some power wires. D'Oh!
I narrowed the problem down to the servo sweep interfering with the drive motors, so for now, I am eliminating that from the sketch.
I will be adding a second HC-SR04 to give this a stereo-like avoidance combo.