Hi,
Here's one suggestion:
Use the CCP module to generate a PWM signal, integrate that signal with a simple low pass filter to get a voltage corresponding to the distance at which you want your rangefinder to "trip". Feed that voltage back into the PIC to one (or both) of the comparator(s). Connect the analog output of the rangefinder(s) to the other input of the comparator(s). Set up the comparator(s) to generate an interrupt when the output toggles.

EDIT: I see that you're using the CCP modules for drivning the motors. Either use a fixed reference voltage or perhaps use the reference voltage module or set up a shiftregister and a R2R ladder or....

/Henrik.