Pierce meat generously with fork. In non-metal baking dish, mix balsamic vinegar, rosemary, thyme, garlic, salt and pepper. Coat all sides of meat in marinade, then place flat and spoon any remaining marinade into center. Roll up meat from short side to make a 10-inch long roll, 5 inches wide. Tie meat several times along a length of roll with string.
Roast for 2-1/2 hours, or until done as desired. Brush with pan juices every 30 minutes. Remove from oven, let cool 10 minutes in pan. Remove string. Slice to serve.
Lamb is done when thermometer registers 140ºF for rare and 160ºF for medium in the thickest part of meat.