Found this on another site on 4L60E:
A hard shift indicates that either the line pressure is too high or there is a problem in the 1-2 accumulator. Was a shift kit installed?
Line pressure that is too high is often caused by a sensor problem (MAF, TPS or MAP).
The proper way to diagnose this is to hook up a scanner and a pressure gauge to the trans. The scanner will tell what pressure is commanded and the gauge will confirm or refute it. A "ATD-5550" pressure gauge is only $32 on Amazon with prime shipping.
The only way to diagnose a 1-2 accumulator problem is to remove the pan, filter and then the dome-shaped accumulator cover which has only 3 screws. Take out the internals and post a picture of it. This will tell us if a shift kit was installed and we will look closely for cracks in the piston.