Transmission won't shift into 4th or 5th gear

hello I have a 94 4x4 v6 Toyota pickupthe check engine light came on while I was driving it 20 miles later it shiFted from 5 into 4.the tranny shifts smoothly into the other gears with no problembut now it won’t go in the 4th or 5thmy transmission fluid is passed a full line if that helpsany help would be appreciated thank you

You have to get the error codes that triggered the engine light. You can try this website for instructions