If you're publishing via ClickOnce, go to the publish page and click on "Application Files". From there you should see a list of your DLL's. Ensure that the ones that are giving you trouble have their Publish Status marked as "Include" rather than "Prerequisite".
What this error message means is that the current publishing settings are going to expect all of the assemblies in question to be present in the Global Assembly Cache on the target machine before installation can take place. Since all assemblies in the GAC must be strong signed, any assembly marked as a prerequisite that isn't strong signed will give you this error.
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