Rough downshifting.
 

I have noticed that when I decelerate that my car downshifts really hard. It makes me a little nervous. I have considered installing a shift kit, but I am not sure if it will help. Any thoughts would be appreciated.

Resistor
 

Right behind the battery is a small resistor. When disconnected, this can result in firmer shifting. However, it comes with side effects... Both downshifting and upshifting can be too firm, and the tranny will voraciously hunt for gears at low speeds.

Make sure this resistor is connected.

You can also check for existing/past TCU error codes here. Simply follow the instructions.