Won't shift into overdrive on 1991 Dodge D150

My automatic transmission won't shift into overdrive but all of my other gears shift correct ly could it be something simple like my overdrive button?

Asked by for the 1991 Dodge D150
Not likely.

Check the transmission fluid for condition and level. A qualified transmission technician will likely need to weigh in on the situation.
Check the Vehicle Speed Sensor for correct operation, located on the driver's side of the tail shaft of the transmission.