Here's a couple more things to check...
Linkages on the end of the sway bar. Sometimes they can be broken and still sit together, and not look broken. This used to happen all the time on older ford trucks, explorers, etc..
when you put new springs in, did you put new bushings in? check that and the shackle on the rear for wear.
Check the pitman arm on your steering box, it could be loose on the shaft
kingpins worn? with the front end off the ground do the wheels have any play?
the track bar on mine has an adjustment on it that had a little play that would clunk while steering or when I hit a bump. I loosened up the allen bolts and turned it 1/4 turn and that seemed to take care of it. also, the end that bolts to the frame bracket has a very sloppy bolt to hole clearance from the factory, so I tightened the crap out of it, and that silenced my front end.
my dad has an 04 250 gasser and he has all kinds of wierd rattles in it. Most of them are actually broken spot welds on heat shields and it drives me crazy... he doesn't even hear them...