I don't really know how to describe this so bare with me. Has anyone felt a little thunk when taking off from a dead stop? It is sort of like weight shifting around in the truck like it would slide forward and then when you take off it slides backward and hits something, I don't fill anything sliding just the hit. It does it proably 1 out of 3 stops some times it is more noticible than others. It seems that the harder you stop the more it will do it which would make since if something is actuallly moving around. I hope this makes since. It seems to be coming from the rear of the truck, you can't hear anything just fill it.

 

It's more than likely your slip yoke...it needs to be greased (teflon grease). There's a TSB out for it, although I can't remember the number. It's covered under warranty.

Do a search...there is all kinds of talk about this.

 

Its the ***** swinging and hitting the underside of your truck.

Its the slip yoke. I have had 3 1st model year ford trucks and they all did it.

Its not like this rear is anything new.