Popping Sound 2001 Ford Super duty 55,000

Unlikely it would be U-joints if you don't notice an issue unloaded. If you don't know how to check U-Joints out, you probably shouldn't try to figure out what is wrong by writing on a forum. I don't mean that to sound wrong, but checking U-Joints is very simple, and only a common sense type of problem. If that is beyond your technical level, you should probably contact a qualified service tech.
From your description, it sounds more like a problem with your fifth wheel simply because it only happens when you hook up your fifth wheel, but you don't provide a lot of information other than it makes a pop when you have your fifth wheel trailer hooked up.