Put the beer, water, kosher salt, brown sugar, and 2 tablespoons of the BBQ seasoning in a large pot. Bring to a boil for a minute and stir until everything is dissolved.Note: Watch carefully as it may bubble up, if it does just remove from heat for a minute and stir.
Let the brine cool (very important) and then add the chicken thighs. Cover and refrigerate overnight (ideally 12 to 24 hours).Note: If the pot isn't big enough for the chicken you can put the chicken in gallon size plastic bags and add the brine.
Remove the chicken, pat dry with paper towels, and season the chicken on all sides with the remaining BBQ seasoning.
Set your smoker or pellet grill to 250 degrees. Place the chicken on the grill grate for 2 1/2 to 3 hours, flipping halfway through. Internal temperature should be above 165 degrees.