This is definitely a rotating mass problem is it not? Brake rotors get so little respect in vibration troubleshooting.
Ok, I am new to the chevrolet scene. I just put my first 2000 miles on an 01 silverado 2wd ext cab., I stumbled across this thread and I can't help but bring up one additional point of interest. But I am in now way new to mechanics, ALL previous vehicles were 1970's models
anywho, I had an annoying vibration problem on my truck, and after much review and money spent, I discovered the problem (in my case). It was the brake rotors. Yep, those shiny circular things that spin tirelessly and transfer your linear motion to circular motion, then to HEAT to slow you down. They get so hot, then you hit a water puddle "JUST ONCE" and they can go out of spec. Sometimes noticeable to the eye.
see:
http://en.wikipedia.org/wiki/Metallurgy
So if its been a while since you packed your bearings with new grease, why not try it. You will need new bearing seals etc. then remove the rotors. Take em to a good machine shop and have them reground or "turned" . Some machine shops can tell you if they are balanced. They drill holes in various sizes and in various lenghts from center to acheive a good balance. I had a set of brand new rotors go so out of shape, I would not have believed it if I had not seen it for myself. SOmetimes having them turned will resolve the problem.
Just something to think about.