First off verify the OD is working, as mentioned above drive to 50 mph and press the OD button on the shifter, you should feel the trans downshift. If not then there is a problem with the button, try some electronics parts cleaner-just a small squirt in the shifter-see if that helps.
If not remove the console top and look for the 2 fine wires, cant miss them. Trace then to there connection, GENTLY disconnect and spray some more electronic parts cleaner to both connections and reassemble.
Road test again, if the tranny downshifts at speed when OD button is depressed the light bulb in the dash is burned out.