Does it slow down, stay the same, or what. The GC's don't like to downshift much while going between say 15-40mph and not a lot can be done to fix this from what I've read.
You can always use the shift lever though.
If the throttle position sensor hasn't ever been changed it could be bad, I replaced mine to fix some intermittent fast idle conditions. Also the trans gov pressure sensor, gov pressure solenoid, and O/D / TC lockup solenoids are easy to change and go bad. I've changed them all in mine and it works much better. Tip don't bother trying to change the harness that comes with the O/D / TC solenoid assembly, you'll spend another hour and decide to just reuse the old one thats halfway built into the transmission.