1994 dodge dakota doesnt shift into overdrive

My dodge dakota will not shift into overdrive. I bought it off of a kid and the torque converter was locked up. so i replaced the torque
converter. it took off fine until i was on the highway for about and hour and the transmission started slipping very badly. but i got it home and did a filter change and put lucas in it. it takes off fine and shifts through the first 3 gears just fine. but it still will not go into overdrive can anyone help? if so it would be greatly appreciated.
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