Place chicken in a 6-qt. electric pressure cooker. Finely grate enough zest from lemons to measure 4 teaspoons. Cut lemons in half; squeeze juice. Add zest and juice to pressure cooker. , Tear basil leaves directly into pressure cooker; add chicken stock. Lock lid; close pressure-release valve. Adjust to pressure-cook on high for 6 minutes. Quick-release pressure. A thermometer inserted in chicken should read at least 165°. When cool enough to handle, shred meat with 2 forks; return to pressure cooker. If desired, stir in additional lemon zest and chopped basil. Serve with a slotted spoon.