My transmission is failing to shift into higher gear

This is most likely one of two reasons. First the fluid is low or dirty. So, let's start with checking the fluid and change it if it is either.

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

Then the next likely reason is the torque converter clutch solenoid is not engaging the clutch allowing it to shift to overdrive.

This could be due to a faulty converter as well however; I suspect the fluid will be dark in color which is a sign that the clutch is slipping.

If this is the case, then we are going to have to replace the converter and the solenoid if that is the ultimate cause of the failure.

I am attaching the info below on how to test the solenoid so let's run through this info and let us know what you find. Thanks