Sobel Wiki is on the Southern Confederation. Typically for Sobel, it's pretty much the opposite of our own world's Southern Confederacy.
To start with, the Southern Confederation stops at the Mississippi. The far side of the river is occupied by the independent nation of Jefferson, which merges with Mexico in 1820 to become the United States of Mexico. Thus, unlike our own history, the South borders a foreign country where slavery is also legal. Per Sobel, about 20,000 white Southerners emigrated from the Southern Confederation to Jefferson in the 1780s, bringing about 4,000 black slaves with them. Sobel doesn't mention any additional emigration from the S.C. to Jefferson, but it stands to reason that there must have been. Jefferson has much more unsettled land than the S.C., so it would be a natural destination for land-hungry Southerners.
The biggest difference between the two Souths is that the S.C. doesn't cling to slavery the way our own South did. Again, Sobel never says that having Jefferson next door made the difference, but again, it stands to reason. In our own history, die-hard white supremacists had only two options: submit to the abolition of slavery and legal equality for freed slaves, or rise in revolt. In the Sobel timeline, they had a third option: pack up the slaves and head west to Jefferson.
In the Sobel timeline, a crippling recession in the late 1830s makes cotton cultivation and slave labor unprofitable in the Southern Confederation. The S.C.'s abolitionists offer slave owners a deal: if the slave owners agree to abolition, the Southern government will compensate them for their freed slaves at twice the going market rate. This deal wouldn't have worked in our version of the South, but for sixty years, the most intransigent white supremacists have been leaving Sobel's Southern Confederation for Jefferson, producing a more moderate populace than our own South had. In the end, the S.C.'s legislature approves the compensated manumission plan, and on New Year's Day 1842, the last slaves in the confederation are freed.