From my internet searches, I have found that the main difference between “hilbillies” and “rednecks” pertain to their geographic region. In a forum on quora.com, one user writes that the term “hillbilly” refers to people who are “hill folk” from mountainous regions such as Appalachia and the Ozark Mountains. The same user notes that rednecks tend to come from flat, farm-land areas who get their “red necks” from long days working in the sun.
The general consensus on the quora forum is that both terms are largely used to imply stupidity or ignorance, though the term “hillbilly” is slightly more associated with this negative connotation. The negative connotation of the term “redneck” pertains towards the particular lifestyle and description of the typical redneck (working class, rural, white, patriotic, pick-up trucks, etc.). Although some people who live this particular lifestyle might take being called a “redneck” as an insult, there are an equal amount of people who embrace the culture and do not see this connotation as an insult.
In my opinion, I feel that outsiders use the term “redneck” and “hillbilly” interchangeably when attempting to stereotype the poor, white, southerner who relishes in the aforementioned lifestyle from the previous paragraph. I also think, however, that there is a large difference between the two terms for people who are familiar enough with both groups to understand their different geographic regions. For this reason, I feel that it is not possible to be both a hillbilly and a redneck despite potentially having similar lifestyles and/or core values.
source: https://www.quora.com/The-American-South-What-is-the-difference-between-a-redneck-and-a-hillbilly