The kickdown solenoid is on the rear of the transmission. It causes the transmission to drop down one gear when you press the accelerator to the floor. If it's stuck, the problem may be the switch, which is located under the accelerator pedal. You probably have to remove the carpet to get to it.

A quick, painless way to test would be to pull out the Klima relay, which is under the hood. Power for the kickdown system is drawn from the Klima, so if the relay is pulled, the problem would go away.