i have the same problem, i wanted my frame replaced a year or 2 ago. checked with the dealer and they said it will cost $15000-20000 to replace it.

so i tried to remove all the rust myself during the height of covid. i removed the rear bumper, tires, rear exhaust for better cleaning and found out that the rear upper control arm that's connected to the frame(both sides) is so rusted and there was a hole in it. i was like, this is not good. it's an accident waiting to happen. i still painted it and removed the rust. not worth it so i made it just a backup car now, never to be driven in rain or snow and just for local roads driving.

to the OP, have those upper control arm checked and the frame part of it. that's where the problem lies.