The divide wasn't really primarily about slavery (although it became such later on), and existed when George III was crowned, if not earlier.
The difference was rooted in the very different economic and social systems in the three regions of the colonies (North, South, and the Western frontier), and was extremely important in the manner which the Revolutionary War was fought (and likely prolonged that conflict by at least a year). The North was primarily driven by pure trading and transshipping of goods due to the many excellent ports (and many excellent places for smugglers to sneak in, which was important due to the idiotic trade restrictions Britain imposed on her colonial possessions) and later those same ports fueled the birth of widespread industrialization, while the South was built entirely on cash-crop agriculture (initially tobacco, later cotton). (The West was mostly subsistence agriculture in this era, and wasn't significantly important to the country as a whole).
The social makeup was also very different, with the South being a very stratified society, with the upper classes owning huge swaths of land that were farmed by indentured servants, slaves, or freemen paid a pittance for their labor. In a very short time, this had evolved into what was an active aristocracy de facto if not de jure, resulting in very little social mobility. Even if a former indentured servant or poor white (or even black, in the earlier days) freeman managed to scrape together the cash to buy the land and slaves for a plantation of his own, he wouldn't be part of Society, and thus snubbed. The North wasn't nearly as stratified (and the wealthiest Northerners weren't as rich as the wealthiest Southerners), and there was much more of a "middle class" makeup the population in the mercantilist days, which endured even after industrialization changed everything. After the IR, while there were massive numbers of immigrant laborers creating a true "lower class" that had no analogue in the South (slaves had somewhat equivalent living conditions, but weren't free, while few poorer freemen were quite that badly off), there was no real attraction to Society at the time, and the only limit to social mobility (in either direction) was the amount of effort you were willing to put in and the amount of money you could get from that labor. (Carnegie famously stepped off an immigrant ship in 1848 and was a wealthy industrialist by the time the Civil War ended in 1865, something impossible in the South.) The result was two very, very different cultures.