Transmission locked in overdrive-96 Diesel P/U?

got a high mileage truck that has had a history of the overdrive gear nut backing off. finally tack welded it to the shaft and it lasted a year.now it won't come out of overdrive-was shifting perfectly till then.- if I can grind the tacks off and get the nut backed off-what is the most likely cause of it not shifting out of overdrive? internal in the transmission or in the overdrive itself which is in the tailhousing section?