a more specific note: the VSS informs what speed the vehicle is moving and works with the throttle positioning sensor to maintain the vehicle speed, since the vss also controls the antilock braking system (which was not noted in having issues along with help of the front speed sensors located at the front wheels), i would lean more towards the throttle positioning sensor, this sensor is located on the throttle body directly across sides from the accelorator linkage). A diagnostic tester should be able to tell if the throttle positioning sensor is performing corrrectly. take the vehicle to your local auto parts store and ask them to read and see if there are any codes present. if so list the code here many of us have manuals that tell what these codes mean.
be sure to list anything you do in details and we will be glad to help further with the issue.