Well a couple months ago I put phosporic acid (Ospho) in a bottle sprayer and sprayed in and around those mounts underneath. I also gently poured it into the cracks made by the rust that those pics showed. Careful though, this stuff doesn't always get along with paint.
This kinda takes it one step further than the oil idea. It converts the rust back to metal and also creates a tough primed surface, but it can only do so much if the rust has eaten away at the metal.
It helped a bit, especially inside the mounts and the channel they are welded to, that I obviously cannot get a wire brush at. I sprayed it like crazy all in there. It seems to have stalled the process, which is about all I can do when it's inside those inaccessible areas. It's also pretty good at killing those roaches here in south texas, hehe.
It's also useful for cleaning rust and rust stains off of stuff, you just wipe it off before letting it dry. Like RWD mentioned this is the only speck of rust anywhere on the whole truck body.
Just another idea I remembered, but obviously you couldn't go spraying all over underneath with this stuff.~Dennis