1998 Ford Explorer speedometer

Ok, I tried changing the speed sensor on the rear axel, but I'm still having the same problem. The speedometer work and then doesn't work. When I took the old one out, it had metal shavings on it. So does that mean I need to change the oil in rear end, and could the metal in the oil be messing up the sensor reading to the point of making the speedometer not work right? Also, what might I try next if that isn't what I need to do, or if it won't work?