I believe the rockers have been replaced. They're solid and when i sanded the drivers side down there was nice fresh metal under the paint. No factory blue paint either so i guess that's a giveaway. I don't know how that right side got rusted so much worse than the rest of the car. Under the car is solid too. No rust on the subframe/frame rails. It's pretty weird.
Edit: didn't realize you said UNDER the rocker panels. I'll definitely check. I wasn't planning on pulling panels off or anything of that matter but i'm definitely going to now. I think it would be best to strip the car down as much as possible to get all the rust out. I want to do this right the first time.