99 F-250 4WD Crew Cab

While driving, the ABS Light comes on intermitently. When it comes on the speedometer drops to 0 and the truck doesnt shift right. Some times it resets itself and everything is fine. Other times the ABS light doesnt shut off. I usually have to pull over and shut the ignitiotn off to reset it. A guy at the ford dealer said it was the rear speed censor in the differential. I changed it, but it still does it. Any suggestions?

Thanks,

Chris