Mix salt, pepper, garlic & onion powders together in small bowl. Rub all side generously with seasoning mix.
Slice yellow onion.
In glass measuring cup or bowl, whisk together BBQ sauce and root beer.
Place meat in large crockpot. Top with sliced onions. Pour BBQ sauce mix over top.
Cover and set to 8 hours.
When done, the meat should easily fall off the bone.
Remove from sauce and shred meat using two forks.
Scoop onions from sauce and add to meat.
Add in approx one cup of cooking liquid to meat and onions.
Serve on grilled buns with additional BBQ sauce.