Not to add to the noise, but to try to help cut through it a bit. Maybe an observation or two from an old Journeyman might help:
I read through everything here and it looks to me like there is a buildup of circumstances that is going to drive everyone nuts until you get enough parts changed. Without taking anyone's side here, the tech may be right about the shocks. Some kinds of shocks (especially "off-road shocks) don't have a lot of 'snubbing action' when they're not being worked in and out a lot. Combine that with a bigger tire/wheel combination and you have a lot of rotating mass that's not being controlled properly. I'm not particularly concerned about the number of weights on any given driveshaft because there are nodes (frequencies) of vibration that each of them is supposed to control. I've bent a driveshaft before (due to my own stupidity and rush to install u-joints) and I can tell you, if it's bent, you know it right now. This was on my Datsun 620, which had a REALLY long driveshaft for its size, with a carrier bearing in the middle.
The other thing that occurs to me as a former wrench is that I haven't seen where anyone has actually spun-balanced the wheels ON THE TRUCK. I would bet that there is something just enough off balance, like a brake rotor for instance, to cause this problem. It's probably not being done anymore because there's a lot of danger in it.
The other thing that seems to be happening is that once this oscillation gets started, there's nothing to damp it so it just gets worse. Hence my comment above about the shocks. They get overwhelmed and can't control the hopping caused by something being out of balance.
Now, maybe one of you guys with the vibration can do the rest of us a favor: Can you get a buddy to follow you on the highway, and see where it's coming from? It should be REALLY obvious, like one wheel hopping up and down. Change lanes and check both sides, front and back. If you can isolate it to one corner, I bet you can get a little more help from the dealer in solving it. We had to do this once with a guy's Cadillac: turns out that the brake drum was way heavy on one side, and the customer's complaints drove us nuts until we found it. When we pulled the brakes down, there was a clean spot that showed a balance weight was missing off the drum. You can't rebalance it; you just replace the drum. And those brake drums had a LOT of heft to them on that car.
So maybe it's not a bunch of marginally balanced brake disks, but that would be one of the first places I'd look if I had a consistent vibration over many cars that wasn't curable by swapping tire/wheel combos.
Long, I know, but hope this helps everyone.
