It's likely the throttle switch. This is part of the modulator solenoid on top of your transmission and if it's not set up correctly you will get a very pronounced clunk as your trans shifts from 3rd to 2nd gear while coming to a stop.

In many cases the actual solenoid is sized up and would need to be removed so it can be lubed and freed up. You can easily check to see if it's working by turning on your ignition and hitting your gas pedal until it pushes up against the kick down switch. You should hear an audible click if it's working. If not, check your fuses and whether you have power at the kick down switch.

Once you have your kick down linkage working you can then adjust the throttle switch so that a test light will go out just after you open the throttle. This will ensure that with your foot off of your throttle the kick down linkage should move to the rear position which will soften any down shifts.