Preheat oven to 325°F. Heat stock in a small pot and add porcini to plump them up.
Dredge meat in salt-and-pepper seasoned flour. Heat a large Dutch oven over medium-high heat. When oil smokes, brown short ribs on all sides then remove to a plate. To the pot, add vegetables, garlic, sage and rosemary; season with salt and pepper. Cover pot and sweat vegetables to soften a bit, stirring occasionally. Add wine and reduce by half; add grapes, vinegar, reconstituted porcini and the stock (leaving behind any sediment that may have settled at the bottom). Bring to a boil then add meat back in and transfer to oven. Braise meat 2 1/2 hours or until very tender.