Transmition shifting hard

this is a common problem with the 4l60e transmission,
you need to have the system scaneed for codes, my guess is you will get a p1870 code,
this means internal slippage detected.

the most likely cause is a valve body malfunction, here is what happend.

the internal transmission line pressure is adjusted by the EPC solenoid, this solenoid is controlled by PWM "pulse with modulation" which simply means that the solenoid is turned on and of many times a second, the longer the on time the lower the pressure the longer the off time the higher the pressure, this on off on off rythem creates a fluid pressure pulse in the system.

now when your on the highway at cruising speed the transmission goes into lock up operation, the tcc valve is in the valve body, the valve is a high strength metal valve and the valve body is constructed of aluminum, as the tcc valve is acted upon by the pulsed high pressure fluid it will somewhat flutter in its aluminum bore and over time will damage the valve bore causing the lockup to slip a little bit.

the pcm detects the slippage, sets the check engine light and in an attempt to preserve the transmission the EPC is commaned full off which in turn puts the transmission at max line pressure and now you have an extremly harsh 1-2 shift.

solution:
have the valve body replaced or allow a transmission shop to remove the valve body **** the tcc valve bore and intall an updated valve.

ps. it will not hurt the transmission if continue to drive it.