I have hard shift and installed a Transgo shift kit into the 4T65-E transmission'* accumulator. BUT...before I installed the shift kit this is what I did to determine if it was the torque converter or the pressure solenoid inside the valve body which are known to be problematic. You will probably want to look inside the transmission pan to see if there are any particles inside there but before u do read on...
First off I had the transmission and all codes scanned are car. Yes in memory it showed the dreaded P1811 long adapt shift code indicating it was solenoid problem.
Since accumulations of varnish can cause the solenoid to stick and check balls inside the valve body to stick, I added one full can (recommend only half a can) of Seafoam Transtune which works awesome to remove the varnish. I drove with this product in, including a few hours of highway driving to get it hot to allow the job of cleaning for 800 miles. Trans Tune | Sea Foam Sales Company
No further major hard shifts like before were present after using the TransTune product. Note with these transmissions it is a good idea to use the transtune anytime before a pan drop for 800 miles, then install new Dexron VI (6) fluid and filter.
Ok so onto the shift kit which is accessible after the transmission pan is dropped.
The Transgo shift kit TransGo | 4T65E: SK 4T65E
comes with instructions and makes sense once you have the accumulator off easy job. I did it in less than 30 minutes taking my time.
Note I also use LubGard Red which keeps the fluid much cooler, in with the new Dexron 6 fluid.
LUBEGARD Automatic Transmission Fluid Protectant