[BNaylor]

I'm assuming you had a DTC error code of P1811 or some slip code which points to the Pressure Control Solenoid (PCS). A launch shudder as you are experiencing could be caused by a flaky PCS. It is not properly regulating line pressure.

The PCS in itself is not programmed. The GM dealer can reflash your PCM module or do a reset to force the PCM into relearn mode for adaptive shifts. However, I do not believe this will resolve your issue.

Replacement of the PCS can be done DIY but not recommended. It is located on the left hand side of the valve body under the tranny side cover assembly (driver's side). The TCC-PWM solenoid is also located there.

Typical tranny shops like Cottmans will replace the PCS for around $500 to include part.

BTW - If the tranny has never been serviced try a pan drop and do the ATF (Dextron-III) and screen filter only. However, I would not recommend a flush either full or fluid exchange.