The rear air suspension uses air bags instead of coils. The spacer won't foot directly onto the air bags. It's been speculated that it can be modified to work, but I've not personally seen it done. Your only option for the rear lift is to remove the air bags and replace with coils or you can adjust the height sensor on the rear to provide lift. This thread gives more info on the air suspension and also shows how to adjust the rear for lift. Any specific questions let me know.
http://www.toyota-4runner.org/4th-gen-t4rs/102272-4th-gen-suspension-options-faq.html