OK, I was thinking about starting a thread on the same topic but since someone beat me to it...
Here are some of my ideas, let me know if anybody has any experience with any of it.
First water down the undercarriage with a product called Salt-Away. Looks like it'll last awhile.
Salt-Away Concentrate Kit with Mixing Unit - 32 fl oz (946 mL)
Amazon has it for $38 if you buy the liquid and spray hose separate.
I also read baking soda mixed with water will also neutralize rust.
---
Then I was going to apply this to the inside of the frame
Eastwood Internal Frame Coating w/Spray Nozzle
Internal Frame Coating w/Spray Nozzle | Eastwood
I ordered this from ebay for $15 a can w/ free shipping.
----
Then I have some Rust Bullet waiting for the rest.
I really love my 4runner and want it to last.
I'll post an update once I have time to apply this stuff.