I think what you want is actually called an under cover.
Part 5140535101
At
2004 Toyota 4Runner Parts - Conicelli Parts Center
I just replaced mine as my old one had several rust holes. It is actually fairly beefy and does a good job of protecting the under side. It isn't as hefty as an aftermarket rock shield, but for me, a mostly road/dirt road/snow, it is perfect.
Most OEM Toyota sells it and you can shop around for best price.