Autonomous 4wd Rover, my first robot. It is a Lynxmotion base controlled by a
Freescale 9S12C32 microcontroller and an ultrasonic rangefinder on a pan and tilt.
Have you tried the Lynxmotion base outdoors (grass, street, forest) ?
How long does the battery last 
An ultrasonic range finder is cool - especially on a pan and tilt - but what does it do… how does it react?