As the problem referenced in this thread
https://grassrootsmotorsports.com/forum/off-topic-discussion/paging-gm-truck-experts-please/277820/page1/
becomes more violent and alarming, I am motivated to find its cause and correct it. In that vein, I have attached a dial indicator to the caliper support bracket and brought it to bear upon the face of the rotor and set a reference point. In turning the hub slowly by hand, I observe that the needle moves almost one graduation on the scale-- which is .001" -- and then back, with the extremes approximately 180 degrees apart. This doesn't seem like a lot to me. What is "acceptable" runout?
Going to check and compare the other side now....
Edit: Okay, other side has definitively more. About 3.5 thousandths. Have I found the problem?
While I'm here, I guess I'll mic rotor thickness and see how much that varies, but the runout would better explain the side to side jerking I'm getting in the steering wheel.
Also going to mark the rotor-to-hub alignment and rotate it 180 and see if that changes anything.
I don't know what GM's guidelines are for rotor runout, but Toyota calls for no more than .002" on the front rotors (.006" is the allowable spec for the rear). You will only feel rotor runout when you are braking, so I don't think it is the cause of the problem you are chasing. Vibration/steering wheel shimmy at 45-50ish MPH sounds like a tire (roadforce) issue to me; have you tried swapping on a different set of wheels/tires?
Runout doesn't tell the full story, depending on the pad material. The more metallic and organic content of the pads, you can have zero runout and still have insane pulsing. My 06 Express van is a good example. My rotors (last time I checked) had basically zero runout, but they pulse so badly that I have to grip the steering wheel depending on where the "pulse" is phased left/right.
The cheap pads have spent 5 years depositing burnt material on the rotors, so parts of the rotor are scrubbed clean while other parts are seasoned like a cast iron pan.
Given the fact that you can probably buy new pads and rotors for $50, it's a cheap parts-throw.
But I agree with Luigi... Shimmy without any braking is unlikely a brake issue. I suggest a tire rotation and see if it changes. Tires aren't perfect, and a wonky belt won't show up on a regular balancer.
3.5 is a problem, but more importantly, two things happen with runout.
As you drive, the high spots 180 degrees out get shaved down when the pads are cold. Tap tap every revolution of the wheel, This causes a thickness variation, which will lead to a pulsation noticeable in the pedal as well as the chassis. This thickness variation takes some amount of miles to develop. Probably kilometers, too 
Until then, the pads (and sliding calipers) follow the rotor when braking and you don't notice anything wrong.
As you drive, the high spots 180 degrees out get extra pad transfer when the pads are hot. Tap tap every revolution of the wheel. This causes irregular pad transfer, which causes a pulsation more noticeable in the chassis than the pedal, and as a bonus it is usually visible to the naked eye as the dark area of pad transfer changing in size, or disappearing and reappearing entirely, as you eyeball the rotor and spin it.
It amuses me when some over-toilet-trained types insist that pulsation is caused by one thing and one thing only. It can be caused by a number of factors, a lot of which can be prevented by ensuring that there is zero runout when the rotors are first installed.
Thank you Curtis and Pete for sharing your wisdom! I do suspect the tires are at least a part of the problem, but was hoping to postpone that purchase.
Curtis, pads certainly I get get for under 50 bucks, but the rotors are massive 2" thick and weigh like 50 pounds apiece. Nothing I'd care to run is less than $70 each. 
On a side note... any exterior accessories? I chased a wobble at 70mph on my van for weeks. Re-balanced the tires twice, did new rotors and pads (it needed them anyway), tire rotations, the works. It ended up being my roof rack. At about 70, it started wiggling in the wind and I could feel the van wobble.
In reply to Curtis73 (Forum Supporter) :
It has some running boards to help step up into the cab, but they're pretty solid. I took it for a short drive just now, and it feels better, but that may just be the placebo effect. 
The tires getting rotated and balanced at the time of state inspection was the very first thing I did to try to track this thing down. Tire rotation had zero effect.
Well I drove the truck a good bit last night. Fortunately, most of that driving was done around 45-50 mph. Truck is fine. Just a tiny bit of steering wheel oscillation at those speeds, and none under braking.
I attribute the improvement to one of two things:
1) I marked the position of the rotor in relation to the hub and re-installed the rotor 180 degrees from its original position.
2) I scrubbed a tiny bit of rust from both the surface of the hub and the inside of the rotor.
