Transmission stopped going into overdrive

At about 148,000 miles the transmission stopped going into overdrive. I bought this from my dad so I know it wasn't beat or anything. I have done the following things to try and fix the problem. Changed trans fluid, new solenoid in transmission, My dad also tore apart dash to make sure the overdrive override switch wasn't blown and that checked out. It has been suggested to me that it probably needs a new Power Control Module (PCM). I'm thinking this is where I go next but at the $168.00 price tag after the $120 core charged returned I wont to get options whether this could be the deal. Another thing that may be helpful is that I drive this truck still about everyday it shifts smoothly and runs great interstate driving without the OD is a no go though. Any help/advice on this would be greatly appreciated. Thx Jon.