My pickup's digital speedometer jumps all over the place from time to time, really fast, from 0 to 185mph to 30mph and so on, and then will settle down for a while, but it usually shows only half the speed I'm really going. It even jumped around when I was sitting still in my driveway. There is no check engine warning light. How do I go about finding the speed sensor and replacing it? Engine is 4.3 V6.