Thanks Ronnie?
The TCC should unlock when you lift off the throttle, no matter the speed. It does so because the ECM thinks you might be braking next. It will re engage immediately upon reapplying throttle above a certain threshold.
Start the engine and make sure it is up to normal operating temperature.
Enter diagnostics. Wait for any codes to be displayed. Then select ECM when asked. The photo below shows the ECM diagnostic screen. While driving watch the triangle over TCC in the bottom left corner. The triangle will illuminate when the ECM is calling for the TCC to lock and go off when it unlocks.
I wonder if it could be caused by a worn engine strut commonly called the Dog Bone? It helps control the rocking back and forth of the engine. It is common for the rubber bushings in it to wear out allowing the engine to move more than it should.
Here is a photo if it. It is mounted between the engine and the radiator support.