[93buicklesabrelmtd]

Pressure Control Solenoid.
These trannys a very sensitive when it comes to the tranny pressure.
Your 4t65e tranny uses the solenoid to control the vaccum on shifts.
The older 4t60e used a vaccum modulator that was attached on the front of the tranny.

Im almost certain this is your problem.
This is a diffrent solenoid from the shift solenoids.
But why didnt dealership suspect that and replace it at the same time?