Overdrive

I have an 89 pickup w/ a 22RE(225,000 miles) and auto trans. A few months ago it just stopped shifting into overdrive. The fluids were new at the time and at the proper level. I changed the fluids and it still wouldn't shift into overdrive. No leaks or any visual problems. Then I had the trans replaced and still the same thing. What could be the problem? A sensor, faulty wires, vac. leak, torque converter? It's still strong from 1st to 3rd, just no overdrive. I've run out of ideas. Can anyone help me?

i dont know if you've been messing around with the dash at all but you could have disconnected the overdrive switch. if this switch is disconected then it wont shift. an easy way to check if your switch is messed up see if it lights up, if it doesnt a sure sign that its not connected, also try turning of your overdrive switch if the dash doesnt display overdrive off then there's your problem

The "O/D OFF" light does come on if the switch is turned off. As far as messing around under the dash, I haven't been under there in 2 years. The problem started a few months ago.