The best way to check it is what you just did. If it backs up a few feet and stops and you have to push the button again then you have a bad speed sensor. The reverser controller runs the starter motor at a reduced speed for reverse (although wide open backwards would be a hoot) and it uses that sensor to cross check the speed. If it is not reporting the controller shuts down after a few seconds. You could also pull it and see if there is anything stuck to the tip of it as it works just like the ABS wheel sensors. Take off the right foot plate and you will see it up and to the back side of the tranny.
