I've got a major clunk in the dually when hitting any kind of large bump (potholes, train tracks) . The truck is new. Feels like the whole rear axle is loose. I've seen the posts and TSB about spring slap. What are the factory torque specs on the u-bolts ( 150ft-lbs?) Are the spring boots called for in the dually rear also? Do they only go on the low overload spring at the bottom of the leaf pack? Could the top overloads be causing this? Should I wedge wood in there to test it? Anything else come to mind?

There will be some noise back there. Lots of springs and they arent really intended to be driven much unloaded. Unless its really bad, I would say its pretty much normal. It woulndt be a bad idea though to just put a wrench on the ubolts to be safe.