Originally Posted by rdruss

Frame rust is a common problem with 4Runners unfortunately, especially if you live where roads are salted in winter. But another aggravating factor that you may be experiencing is that the AC drain in the engine compartment drains right on top of the passenger side frame rail, the condensate runs down the frame under the passenger side door, where it stops and starts dripping on the ground. But it starts rusting the frame where it stops. You need to run an extension hose from the drain hose Toyota has there and route it to the outside , away from the frame.

My frame had a 8" long x 2" wide hole in the bottom of the frame rail from this and i had a couple of steel plates welded in at the cost of $1000. Yours doesn't look too bad, even with the hole, but take a screw driver to the bottom of the frame in that area and see if it's still solid or you can poke through. If you can poke through may as well knock all the rusted part out of there and have it welded like I did.