What year did the Arizona Cardinals join the NFC west?

2002.

The NFC realigned from 3 five team divisions to 4 four team divisions in 2002. The Falcons, Saints, and Panthers Left the NFC West for the newly formed NFC South division, the Cardinals moved from the NFC East to the NFC West, and the Seattle Seahawks moved from the AFC West to the NFC West. The 49ers and Rams stayed in the NFC West.