It sounds like a ground issue simply because it shifts normally with the scanner attached. The only thing different is you are providing an additional ground path for the computer.
Do me a favor. Check on the rear of the intake. There is a ground strap located there. Make sure it is attached and in good condition. I believe it runs to the firewall from the engine/transmission bell housing.
Next, I attached the wiring schematic for the entire automatic transmission circuit. I highlighted a ground I would like you to check as well. I had to cut the pics in half to make them readable, but I did overlap them so you can follow from one to the next.
Here is a link you may find helpful:
Let me know what you find.
See pics below.
Images (Click to enlarge)
Sunday, June 13th, 2021 AT 6:57 PM