Ive recently noticed that if I put my truck in either drive or reverse and take off quickly like somewhat punch it I here a clunk same as if I'm backing up a hill. It also does it sometimes if i come to an abrupt stop. Its a 99 5.4 4x4. I'm thinking u joint?

 

That's the first thing to check.

 

It could also be the slip joint in the driveshaft. They can get dry and stick. Ford dealers sell a special grease for this.