rear speed sensor problem on 1998 Dodge Ram Van 1500

incorrect voltage from rear speed sensor when vehicle moves over 15 mph. i changed the sensor and removed the code and the code came back over 15 mph. what should i do.

Some times when I have had this code, it is because the read differential is full of worn metal and the metal particles stick to the Speed Sensor. Was the old one covered with little metal particles? If so, you need to replace the rear diff fluid. I usually do this when replace a rear speed sensor on most trucks.
Did you clear the ABS code? This must be done with a scan tool, disconnecting battery won't do it.