Hi John,
You might have some moisture inside the tranny that has frozen (ice) and will not allow the shifter to come out of reverse. I would suggest warming the tranny case and try rocking the output shaft back and forth while pulling on shifter with moderate effort. If this is unsucessful you could drain the case of all oil/water, since the case is warmed up now. Fill the case with diesel fuel and let it set a few days and try again.
You also might try removing the shifter tower from the top of the tranny and make sure it is not bound up in the shifter itself.
Good luck, Carl
