Trim fat from pork roast then place roast in a slow cooker and pour broth over top. Top evenly with onions, thyme rosemary and lightly sprinkle with pepper. Cover and cook on LOW 8 - 9 hours or HIGH 4 1/2 hours.
During last 30 minutes of roast cooking, in a medium saucepan whisk together balsamic vinegar, ketchup, honey, brown sugar, worcestershire, mustard, garlic, 1/2 tsp pepper and season with salt to taste (keep in mind pork is usually pretty salty so you won't need much). Bring the sauce to a boil over medium heat, then reduce to a simmer and cook, stirring occasionally, until slightly thickened about 20 - 25 minutes.
Remove pork from slow cooker, shred while removing any fat. Drain onions and broth from slow cooker and discard then return shredded pork to slow cooker (set slow cooker on warm heat at this point). Pour in balsamic sauce mixture and toss to evenly coat. Serve in buns with coleslaw if desired.