How many miles? You probably need the transmission fluid flushed.
Bring it to a mechanic that knows the difference between good transmission oil and the stuff Valvoline and other quickie places use.
---If your engine has over 50k miles, don't flush the transmission. That will cause blown gaskets leading to leaks. Your transmission being as old as it is, cannot take the full pressure from the fluid flushing.
Shifting too hard is a common aging problem with this 4T65E GM transmission. The 4T65 is found in many light to medium duty front wheel drive GM vehicles from 1997 to 2003 such as Pontiac Grand Prix, Chevrolet Lumina, Chevrolet Venture, Chevrolet Impala, Monte Carlo, Oldsmobile Intrigue.
The problem is resolved with replacing the electronic pressure control solenoid. In most honest transmission repair shops this will cost about $800. What happens is over time the EPC solenoid and it's bore (all within the valve body) gets stuck in a high or low position. This stuck valve will set a consistently high or low line pressure feeding the other shift solenoids too much or too little pressure resulting in all gears shifting with incorrect pressure.
In some cases where the vehicle has over 150k miles and the auto body and interior are looking a bit old it may be best to just drive the vehicle, no harm will be done. Otherwise, if the vehicle is in good shape the $800 to repair the transmission could be great news for the owner and passengers! http://LealesTransmissions.com