Washing and spraying your frame off to keep this under control isnt really a solution to this frame rust problem. These frames are rusting inside out in the same area. Salt is collecting in that area of those rear brackets. Not much you can do besides attempting to spray water through those little holes in the frame to move the buildup around. Unless you dumped oil down your frame since new or didnt drive it much in the winter months you too probably have rot...its just not through your frame for you to see yet
Really?
Out of curiosity I used my borescope to look inside the framerails of my 17 (nearly 18) year old 4Runner, which spends it's winters on salt-encrusted roads. You know what I found? No rust.
The vast majority of people don't bother to maintain their vehicles. The fact that unmaintained 4Runner's can last so far beyond the rust warranty period, is pretty impressive IMO.