create or replace function arrxor(list1, list2) as
  (select array_agg(elements) from
    (select min(arr) as mn, max(arr) as mx, elements from
      ( (select 1, unnest(list1))
        union all
        (select 2, unnest(list2)) ) as t(arr, elements)
     group by elements
     having mn = mx) ) ;

# Example:

select arrxor(['this','is','a','test'], ['also','part','of','a','test'] ) as arrxor;