Ive experienced that if the parking brake is engaged, even a little, that it will prevent turf mode being engaged and keep the rear diff locked. If its not engaged, I would look into the sensor on the parking brake. .Yeah mine is not going into Turf Mode either, I don't think at least. When I make a sharp turn in the gravel you can tell it seems to dig a little on the inside wheel. I read something in the service manual about a sensor or something, that if not working correctly would not let Turf Mode engage. I plan to work on that too. I did read that the actuator on the rear diff activates for Turf Mode, and is off for regular locked mode. Maybe start by checking to see if that actuator is getting power when it should be?
No idea there, sorry.