I have the exact same problem and I am going to install Firestone air bags... you can go to The Diesel Stop forum and do a search on "shudder" and find lots of threads about it... I am including one here (even tho the title of the thread is blaming the tranny):
http://www.thedieselstop.com/forums/...r-load-192294/
I have talked to people who have had Ford shim the carrier bearing only to have the shudder occur when empty.. I have also talked to those that have not had the problem no matter what load they are carrying.... so rather than shifting the problem or dealing with Ford, air bags will be my (hopefully) cure..
I am with you Ford should pony up for the proper fix, but that ain't gonna happen... the softer spring pack and the longer springs in the 450 pickups to give us a less "commercial truck" ride, I feel, is the problem..
Good luck..