Truly, I do know the difference. But maybe Southerners use them more interchangeably than others, and guess what- thankfully, there are those with more credibility than I who've decided it doesn't matter- that they've come to be so confused and exchanged that it's now acceptable just to give up on the differences.
Like this blog entry, from The Word Blog. citing several dictionary entries.
The OED says that farther is usually reserved for use as the comparative of far (ie. measureable distance) while further is applied to figurative, unmeasurable distances or extents like time or metaphorical distance.
CanOx says that farther is simply a variant spelling of further.
Fowler’s Modern English Usage speculates that farther will become less and less common until further becomes the universally applied term.
The Chicago Manual of Style supports the distinction of meaning as set out in the above definitions.
And Merriam-Webster’s also agrees with the above definitions.
But maybe I'll go with Fowler, stick to further, and be done with it. Just the other day in an interview, John Grisham used further when he meant up the road a piece, and nobody batted an eye. Oh, wait, he's from the South.
So, is it a Southern thing, a new convention, or just plain incorrect grammar?