Frame doesnt really have to be bent to cause this. It doesnt take much "diamond" in a truck frame to cause what you are seeing. And diamond doesnt take much of a jolt to cause.
This is how you tell if you have "diamond".
Look at your front bumper and see if it sits a little further back on either side. It could be as little as 1/16" or 1/8". If so then you need to take it to a body shop that has a frame machine and get it fixxed. It can improve tire wear and fuel mileage by getting it fixxed.