2001 F150 5.4L 4 x 4 No Reverse
 

Got in truck today, engaged 4 x 4 drove around a while.
Parked truck. Got back in truck, started up, put in reverse
Very Loud Metallic Clunk
No Reverse
Put in 4 x 4 no reverse
All Forward gears work fine, drove straight and around in circle to check operation and all was fine
Still No reverse
When I put it in reverse it sound like something engages but it goes no where

So what is the most likely thing that broke

Thanks

Take a look at this article.

https://www.ford-trucks.com/how-tos/...reverse-356889

That article does not address the Loud Clunk
No Lights No Codes and shifts into forward just fine

To diagnose the problem, you'll need to do the diagnostic steps outlined in the article or take it to a shop for a diagnostic. Simply knowing there was a noise isn't a diagnostic that will tell you the underlying problem. It simply confirms something happened and there is now a problem.

replaced
 

I replaced the transmission, took the old one apart and found the reverse band where the servo pushs in busted off IE the loud clunk