I have a 2009 F-150 with the 5.4 engine and 145K miles. In the last month or so, when I am cruising at speed there is a cloud clunk and the truck will jerk. Sometimes is a minor jerk and sometimes it feels like I've been rear ended. The engine never loses speed or RPMs. It will go a week without doing it and then maybe do it 5 times in one day. There is no way to predict when it will happen. I'm not sure where its coming from as I do not know a lot about vehicles. Spoke to a mechanic who says it could be a transfer case chain slipping. Said there is no way to tell for sure without taking it apart and its an expensive job. Does that sound like a chain slipping. In the last 3 months I've had to replace the U-Joints, and I've greased the slip yoke. I've also had the rear end checked and it is working fine. Thanks.