Wondering how the repair went. I've been prepping the frame/undercarriage on mine for a fresh coat of paint and found the same situation. Other than the (2x) 6-8" x 1" areas that are rotted out on the inside of the passenger side rail, everything else seems to be solid.
Well that seems all too familiar!

:
I guess I forgot to post a follow up, so I'll do that now!
The repair went well. I took the 4Runner to a professional welder to get it patched up. To prep, he emptied the piles of loose rust inside the frame (which likely held water, accelerating further rust) and sprayed a rust inhibitor.
He patched the passenger side frame rail in 3 separate places, each patch much larger than the damaged area to ensure the patches were welded to solid frame material. The steel used is similar in thickness to the frame steel. Two of the patches were flat steel plate on one side of the frame rail, but for the rear section that supports the control arm, he wrapped the rail. A single sheet of steel was bent to cover the outside, bottom, and inside of the frame around the rear control arm mount. Additionally, most of the perimeter of the mount was welded to the new material.
The patches were finished with some kind of rust preventative spray paint (not sure of the brand used) and drain holes were drilled.
While I'm pleased with the work and my concerns about structural integrity have been appeased, I would have preferred to cut out the rust entirely. The welder advised against it, because it would add significantly to labor costs with negligible difference, and that this truck probably only has a couple good years left in it anyway.
Obviously convention is to _always_ remove rust before repairs... Assuming I want to keep the truck for more than a couple years, I'm thinking this plating could be a good temporary fix. I'll revisit the problem in a year or so, when I have a garage to work on her myself :boxer:
Loose rust from inside the frame:
https://flic.kr/p/N9nuAW
Plate welded to bottom section (mid frame):
https://flic.kr/p/MKedCb
Rear frame and control arm mount:
https://flic.kr/p/MKebVo
https://flic.kr/p/N9ny6m
https://flic.kr/p/N9nznQ
https://flic.kr/p/N4SqWP