I have a 03 cal. truck that has a hard clunk when I put into reverse. It does not do it all the time, only after the truck warms up. I can put it into reverse and 2sec later it will fill like a backed into something. I have asked other people if their trucks do the same thing, and they tell me no. The problem started after the first week of driving the truck. So I know that it is nothing related to what I have done to it. If anyone can give me some insight on what it might be that would be great.