2nd Gen Automatic Transmission Don't Shift Into Fourth Gear

[Brix&Filip]

Hi,
My '91 T4R doesn't shift into fourth gear. I have no codes on the OBD1 and O/D light turn on and off. Any ideas ?? Can it be a solenoid ?? Truck has 45K. Spent many years parked.
Any ideas will be greatly appreciated.
Thanks guys !!!