Is this a new concern? Have you tried doing anything to correct it as of yet?
The Colorado trucks have a known issue with the tires wearing unevenly. Since you've already done the alignment, I would start with the tires. Try rotating them first. If the shake goes to the rear, then you know it is one of the tires that was on the front. A low speed shake or shimmy like you are describing is most likely a shifted belt in one of the tires. Balancing the tires won't help a condition like this.
If you have a shop nearby that has a Hunter Balancer that can measure road force variance, you could try that as well. This machine will measure the tires to make sure they are "Round" and not egg shaped. This should help you pinpoint which tire is causing the problem.
I hope this is the answer you needed. If not, let me know.
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I apologize. You didn't mention that you had done that.
Does the vibration seem like it is wheel speed related? Is it more of a ujoint type vibration that only happens when you take off from a stop?