Yeah, I was brought up in the midwest, snow belt, and I've seen frames rusted through. But I live in a salt free part of the world now, so I expect more, and I'd like this vehicle to last me 10 years or so, and be able to work on easily.
This is what a 2019 Land Cruiser would look like after maybe 3 months of its first winter in Chicago. I was at my brothers Lexus dealer a few weeks back undercoating his brand new 2020 GX460, just as we were getting finished up, one of the techs pulled in a 2015 LX570 with 50k miles, local Wisconsin vehicle. While far from a rust bucket, it had MUCH more rust than the vehicle you posted.
Looks pretty decent for an OR/WA truck depending on the area. Stuff on the diff is pretty much just surface rust, actually most of it looks to be surface rust. I would take a wire brush too it to scrape the scale off then I would go at it with some POR15, Chassis Saver or something like that to help neutralize and kill it and then paint over.