Truck Repair: vibration, goodyear tires, chevy silverado

Question
I own a '00 chevy silverado,5.3l auto,reg. cab shortbed 2wd.I bought it about 7 months ago and since day one it has had a hiway speed vibraton that is intermittent.The factory wheels were traded out for boss 20x8.5 wheels and goodyear tires.I have had them balanced 3 times by different shops and while it has seemed to make the vibration better at times there is still something wrong.I have also had 3 different shops check the alignment and wear of any components.On any given day the truck can be driven at any speed and there will be no vibration or what I call "shake".The very next day it will feel very unnerving from 55 on.This became very confusing a few weekends ago while on an interstate that had a 70 mph speed limit.With the cruise sat at 70 the truck would constantly go from vibrating to being ultra smooth every half mile or so and consstantly did so for several miles.I've had th etruck to a reputable collision shop and it had no signs of being wrecked and the truck drives straight as a pin on a good road with no unusual tire wear.Any help or suggestions would be greatly appreciated.

Answer
Hi Derek,
I am wondering if the vibration might be from engine rpm's rather than from something rolling. Check by puting it in neutral when it is vibrating, and let off the throttle, and see if the vibration continues.
One possibility I can think of if it is related to rolling, is check the U joints, and see if one might be slipping to one side, and allowing the driveshaft to be off center.
Van