In a small saucepan, combine 1 cup water, the kosher salt, and brown sugar. Bring to a boil and stir until smooth, then remove from heat. Allow to cool for about 10 minutes.
While the salt mixture is cooling, combine the cold water, tequila (or citrus juices), orange zest, lime zest, orange juice, and peppercorns. Add the salt mixture and stir to combine. Submerge the chicken thighs in the brine and refrigerate for 4-24 hours. Halve any remaining citrus and reserve.
When ready to cook, preheat grill over medium heat. Lightly oil the grates, then cook the chicken thighs for 4 minutes per side. When you flip the chicken add the reserved citrus, cut side down. When cooked, remove the citrus from the grill and transfer the chicken to a serving plate. Allow the fruit to cool for a few minutes, then squeeze the grilled fruit over the chicken and serve immediately.