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So, I see you've got your hands really full there. But it's not insurmountable. The underside doesn't look all that bad. The joists are resting on a beam, which is supported by posts on footings. For the most part, I'd say you just have to jack the low area up, check it with a 6' level, and replace the posts with longer ones, preferable pressure treated. You can place the jacks right on the footing next to the posts.
I wouldn't go tearing out the floor if you're able to get under there. It would be a lot of work crawling around in the dirt, but then you wouldn't have to replace the floor.
Will you be doing this yourself, or at least one other. I ask because it would be ideal to have one person underside doing the jacking, the another up top checking for level.