From your description and the fault code you mentioned your 'bucking' may be caused by the tranny slipping and grabbing. If you have a tach, check if the RPMs climb and suddenly drop when the car bucks. Be careful with this as it's really hard on your tranny and engine mounts! The range sensor is a $35 part at most dealers and is locatated on the top of the tranny. About a 45 minute repair on the V6 which involves draining the cooling system and disconnecting several hoses. Not sure about the Zetec??
Check this thread which has a pic of the tranny and sensor:
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=tranny&Number=365232&page=1&view=collapsed&sb=5&o=7&fpart=1 
