1999 Dakota ABS - BRAKE light on

I have a 1999 Dakota R/T with 65000 miles on it. I was going to work one morning when my ABS and BRAKE light came on and stayed on. I noticed that my cruise control doesnt work either. I checked my codes and came up with a P1687. My speedometer and all my other gauges work fine. I have heard that it could be my ABS sensor on the rear diff. I dont want to spend money on a new sensor if thats not the problem. I would really appreciate the help.

Thanks
Tom