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According to the vendors, the bumper brackets for Chevy and GMC are the same. Are you sure that the bumper you're trying to mount is an original? Those brackets in your one pic I can see look very much like mine.
[on edit] I was able to open your photos (leaving off the first http://) and brackets in your first photo look nothing like the ones that would be correct. I think your brackets and bumper that came on your truck are not original.
By the way, your image links didn't work (even when I pasted them into a browser window.) In order to post links to pics out on the web, you need to use the link icon (the little world/chain icon). Or even better if the image is on your computer use the Attachment manager (visible on the lower portion of your screen when you're in the full editor mode when posting.) Full instructions are in the box on the lower left of your screen when viewing posts.
Also you should post an introduction to yourself and your truck in the
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