My seat was doing the same thing.. clunk and movement around corners braking..
Argued with the dealership, they kept telling me it was normal.
Finally I went in and showed them it wasn't normal and it turns out the motor for the seat on the front driver side(door side) was missing a bushing which allowed the motor mount to have alot of play.
I guess they forgot to put it on. I'm guess my truck was built on a friday of a long weekend. I hope this isn't a clue of what is to come.