Grill chicken for a minute per side in a grill pan or on your outdoor grill. You want the pieces to just firm up and seal in the juices and get some char marks on it. This is not to fully cook them. Remove and set aside.
In the bowl of a 6 quart slow cooker, mix the BBQ sauce, vinegar, pineapple, red onion, butter, oil, Worcestershire sauce, and brown sugar. Give it a good stir or two.
Nestle in the lightly grilled chicken pieces and then stick the jalapenos throughout and cover to cook.
Cook on high for 3 to 3 1/2 hours. If you prefer to cook on low then cook for about 6 1/2 hours, or until chicken shreds easily and is still moist.
When done, remove chicken to a sheet pan and shred with two forks.
Meanwhile, slowly and carefully, and using oven mitts, pour the sauce from the crockpot into a sauce pan, remove as much of the top oil, and boil on medium-high for 20 minutes until it is thickened and reduced by almost half. Stir occasionally so nothing sticks to bottom of pot.
Add shredded chicken back to the crock pot on warm and pour reduced sauce over it, and combine. If you are not shredding your chicken then just pour the reduced sauce over the whole pieces.