Do I have a case?

The question

I purchased a used truck about 1.5 years ago from a dealer. When asked about prior damage if any the dealer told me is was only minor. The car fax report only shows minor as well. While I was under the truck checking the size of the spare tire I noticed there is frame damage on the left rear. I have reached out to the dealer to notify them as they never disclosed this. They are saying they didn’t know and the auction never disclosed to them. My main question is, is the dealer or auction or either of them responsible and do I have a case to pursue? The title is a clean title, not salvaged. It is financed through a bank. But I’ve see. It before when these dealers buy cars with known damage and never disclose it or submit it to insurance, then “fix” it and sell it. My main concern is the integrity of the frame and how the vehicle will hold up in a wreck. I have a medically complex toddler that is already fragile and this is the main transportation used. Thanks for any advice.