The Arbitrary ChuckJerry

Here's a line from the Declaration of Independence:

"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the Pursuit of Happiness."

To say that "all men are created equal" seems to me to be grammatically incorrect. The "equal" part is an adverb, it is describing the verb, specifically how men are created. As that is the case, shouldn't the phrase be "all men are created equally"?

I may be wrong here. Not in the sense that the adverb should be "equally", but in the sense that the "equal" may not be an adverb. This phrase may mean something like "all men are created as equals" and the "equal" is therefore some other part of speech. I can't think what part of speech that may be, however. It is in the obvious place that an adverb should be and fits the context of the sentence, but as written it is clearly not an adverb.

Any thoughts? Lawyers out there, is there an accepted meaning in this line?

Incidentally, this is a beautiful thought and is brilliantly worded, despite the perceived error. If you read this for what it's worth and its intended meaning, the ideas expressed here really lay out what America is supposed to be all about. It's too bad we as Americans don't have a history of upholding these values.