Oh, darn! That shift kit seems way too harsh. Do you know what brand of shift kit they put in it? If the shifts are harsh like that, they are putting extra stress on internal parts like drums and such and can actually cause premature damage.
Hopefully it's only things like stiffer springs on the valvebody. The thing is that different manufacturers do things differently to get the same effect, so unless you are an experienced tranny rebuilder, you likely won't know what you are looking for.
Here's a sort-of-common shift kit from Transgo. Most of the parts go into the valve body, but the set of springs that you see on the top I'm pretty sure you have to take the unit apart to revert to the OEM springs, as they go into various drums. Some parts you want to leave in, as they address common issues.
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I don't know much about these units to tell you what to change back to get rid of the harsh shifting. What year is your truck?