The "black plastic box on throttle body" you referred to is the throttle position sensor. Rearward of that and mounted to the intake is the Idle Air control valve. It is mounted with 2- 8MM screws (i think, could be 10's though) If you still have the black plastic cover with 4.0L lettering covering the throttle area, it snaps around the IAC.
Hope this helps
Dialtone