When the OD light flashes it means you have a driveline code that has been set. You need to have your codes read, resetting does nothing but make the light go off for a little while. Is your speedo working properly? The reason I ask is one of the most common driveline code is the VSS on the rear diff, cheap and easy to replace, but first get the codes read. Our trucks aren't OBDII because of GVWR so don't trust the free reading from AZ, or Pep Boys, etc to read your truck.
NCH