Im liking these ideas. It does sound like a load/ unload noise.
Try a can a spray grease and just hose the springs and bushings and hangers and see if the noise changes.
I still dont think its Pinion
I don't think it's pinion related, either.
What kind of shape are the U-bolts & lower plates in? A notorious 400 problem area.
U-joints good? I would think that those would of been one of the first things checked.
Leaf spring centering bolts still there? They haven't broken or just turned to dust? It happens.
How's the body mounts?
Tried rocking the vehicle not just up-n-down, but side-to-side?
Spraying some lube is a good idea. If you have the patience, just spray one element at a time, sayyy...every second day. This'll make it easier to narrow down the exact problem.
I'd start with the lower shackle bolts/bushings. Just one at a time. Followed by the uppers, then move to the front hangars. Upper/lower shock bushings, etc. etc.