My problem started out as a surge when the cruise control was engaged. I noticed that the speedometer needle fluctuated slightly at that time. The fluctuation got worse to the point the cruise control wouldn't stay engaged. I drove a little farther and the ABS light came on. I stopped for fuel and when I took off the ABS light was off. However, the overdrive light on the shifter was flashing and the truck shifted hard. The speedometer needle was steady at slower speeds, but at highway speed it jumped between 40 and 80 mph. Do I have a speed sensor that has gone bad? Is it the transmission speed sensor or is it the ABS speed sensor that is in the differential? If you all decide it's not the speed sensor at all, what should I be looking for? Thanks for everyone's help.