[stan_rogers]

Responding to your edit: no, the rules of AAVE nearly all originate in non-standard dialects of British and Irish (Hirberno-) English. They aren't something that spontaneously arose from a deprived economic climate or a dysfunctional society. There are only a very tiny number of differences between AAVE and American Standard English that can be explained by an element of weak creolization (such as the dropping of to be in statements of identity and optional inversion in forming questions); the rest find exact parallels in dialects on the ground throughout Britain and Ireland. In fact, most Newfoundlanders would have little trouble with the grammar (though not necessarily the slang) of AAVE, since their native dialect derives from a very similar mixture of British and Irish dialects. While it can probably be fairly stated that the colour bar has been responsible for the isolation of the black vernacular grammar (and its failure to transition to something closer to American Standard English), it is not dissimilar to the native English dialects of the people who were once working alongside them as indentured servants rather than as slaves.

The idea that diglossia (or triglossia) does not exist in the mouths of educated, and even privileged speakers of English (wherever they may live) is preposterous outside of a relatively small part of the American socioeconomic strata (and by American, I mean North American — much of Canada is weirdly homogeneous as well). Most native speakers of English speak two or more "Englishes", each with its own grammar and vocabulary. AAVE is not a "lesser" dialect; despite its speakers often being disadvantaged, the language itself is no less legitimate. However, like most of us, in order to move outside of their dialectical grouping, speakers of AAVE must also have command of the prevailing standard — just like Jeff Foxworthy's redneck brain surgeon.