19 year old post, you will be lucky if 250xRKrider is still here.

If you have no speedo, how do you know the sensor is out? Also a bit puzzled by OPs ref to not being able to change down. A 2000 Rancher would be a 350, so non auto, could be an early ES, but I would be surprised if the speed sensor affected gear changes on them. Though come to think of it, I vaguely remember a faulty speed sensor wire causing gearchange problems on a 420ES, but faulty ES buttons, angle sensors and shift motors are all more common ES problems.