Yes, reference sources say it's used more in US than UK. It was recorded as a Suffolk term in 1823. Isn't Suffolk supposed to be one of the main population sources for the early south? I have a vague recollection from Albion's Seed.

OED:
A fat or oil used to make pastry, etc., short.

1796 A. SIMMONS Amer. Cookery 34 Loaf Cakes No. 2 Rub 4 pound of sugar, 3 and a half pound of shortning, (half butter and half lard) into 9 pound of flour. 1823 MOOR Suffolk Words, Shortning, suet or butter, in cake, crust, or bread. ... 1970 SIMON & HOWE Dict. Gastronomy 347/2 Shortening, a culinary term used more in the United States than in Britain and it applies to fats used in making breads, cakes, pastry etc. All fats, even oils, come under this nomenclature and are used because they make mixtures 'short' or tender.