my 4x4 van front end had a serious case of the rattles, got progressively worse, before I pulled it completely apart the first week of December for a front end rebuild. In fact, I'm procrastinating going outside this morning, by looking around on the forum!
After my experience, the last thing I would suspect is the leaf spring packs. Not completely off the table, it's just there are a bunch of other more likely culprits.
In my case, I had several things going on, they all made noise while out on fire roads, even on the highway driving over overpass transitions and road seams.
Stub axle bearings:
Using a pry bar or large screwdriver, check to see if the stub axle has a lot of up and down movement (gently pry up on the ujoint, against the inner C). There's a small needle bearing that is infamous for going bad. Mine had 3/16" movement, were so bad the ujoints had been bouncing off the lower ball joint (the rattle), and had left a witness mark. The inner dust seals were chewed up, from my internet research, those are both tell tale signs. Mine were bad for some time I guess, when the inner needle bearing gets destroyed, it chewed up both stub axle bearing surfaces, a common issue.
gently pry on the sway bar at the link ends and bushings. I'm not familiar with what style SMB uses, but my upper frame attachment bolts had loosened up, and were rattling. My bushing were shot, too, the bar was rubbing against the steel bushing bracket (that looks like an electrical conduit clip on steroids)
I've got a 2001 Dana 60, but I understand all the modern 50 and 60 use a non-servicable unit wheel bearing and hub assembly. I think, but not sure, SMB offers a Spyntec or Dynatrack hub option, that IS serviceable. Either way, to check them, jack up the front end one side at a time, by the axle housing. Shake the tire from the 12 and 6 o'clock positions, there should be no wheel bearing play. My passenger side squeaked like the bearing was audibly dry, and had a dangerous amount of play. Drivers side was good.
tricky to check, but famous for only lasting 50k-60k miles. I supose less if you traverse washboard fire roads. Be sure you don't tear the grease boot in the process. Pry upward, between the knuckle and the inner C. Mine checked out fine, until I puled it aprt to repair the other stuff, I found 1 ball joint had a small amount of play, 1/32", so I replaced all 4 with Moog greaseable balljoints. It's a big job.
also check the shock and steering damper bushings, the tie rod ends, pitman arm, steering box bolts being loose.
I hope that is helpful, Geoff
1995 E350 7.3 Diesel, 4x4 high roof camper, UJOR 4" lift