It's actually from a Scottish word, craic, originally used by the white Scotch-Irish population to mean "talk" or "yak." Here is a quote from the web site Scottish Hillbillies and Rednecks:
"..."craic" (Crack) is a term still used in Scotland and Ireland to describe "talking", chat or conversation in a social sense ("Lets go down to the pub and have a craic"; "what's the craic"). The term, first used to describe a southerner of Ulster-Scottish background, later became a nickname for any white southerner, especially those who were uneducated."
Here's another resource: Cracker Culture: Celtic Ways in the Old South
FL Cracker Houses becoming a popular architecture: Florida 'Cracker' Becomes Hot at Upscale Developments [FR]
In any event the perpetuation of the word would not have continued without a more common understanding of it's usage.
It also says Redneck means Presbyterian dissenter.
The real meaning of Redneck is a dirt poor farmer that is out in the fields all the livelong day until his neck is burned so red that Angels cry and the Devil takes credit.