Chevy & GMC Vans

I just got finished fixing some other stuff and now I am having overdrive issues. It will shift into overdrive, but will not lock out like it used to. I'm only getting about 12-13 miles per gallon and I think I should be getting around 16-18 on highway.

Any suggestions? Fluid is pink and full. No other shifting issues or problems. I did used to get a error code on the computer TCC stuck on but hasn't appeared lately. I am assuming that there is a solenoid that can be changed with the pan off?

Yes you can get to the torque converter solenoid with the pan off. And based on the code you were getting that sounds like your problem.
Aside from gas mileage, your trans will run hotter without the TC lockup.