Is the outlet vent position set to windshield/windows only? Recirculate is not selectable when set to windshield/windows mode--it's locked in fresh air mode.
In windshield/windows and bi-level, you can select recirculate. On mine, it will turn off recirculate if you press windshield/windows, but if you press it again, it will switch back to the previous mode and turn on recirculate if it was previously on.
Not sure when Toyota started this change, probably vehicles that began a new generation after 2003, like the 4th gen 4Runner. My '04 Camry allows recirculate to be turned on in the windshield/windows mode. My '05 Tacoma, like the 4Runner, doesn't.