Why doesn't my 1989 Fullsize Bronco shift into 4th gear or OD?

that sounds like you may have a bad shift solenoid in the transmission not allowing the trans to shift to 4th gear and therefore you will also not have direct torque conveter clutch lockup also. They can be located on the valve body of the transmission or elsewhere is all depends. Depending on the trans that can be a easy fix or a more difficult fix im really not sure for the broncos.