In a small bowl combine garlic, coriander, cumin, oregano, onion powder, salt, black pepper and cayenne pepper; rub into the meat.
Place meat in a roasting pan that has a cover; add beef broth.
Cover and roast at 325F for 2 1/2 to 3 hours or until very tender.
Remove meat from liquid with a slotted spoon; discard excess fat from cooking liquid, reserving the liquid. When meat is cool enough to handle, shred it using 2 forks pulled in opposite directions.
Stir in 1/4 cup of the reserved cooking liquid to use as taco filling. Or, stir in enough cooking liquid to moisten; reheat in a saucepan over medium heat, stirring frequently, and serve on toasted Kaiser rolls.
This can also be prepared in a crockpot with equally yummy results:
Rub the spice mixture into the meat, place into crockpot.
Add beef broth.
Cover and cook on low for 8 to 10 hours or on high for 4 to 5 hours.