Let the digits be x and 2x. The difference between the numbers is $|(10(2x)+x) - (10x+2x)| = |21x-12x| = 9x$. Given $9x=36$, so x=4. The digits are 4 and 8. Their sum is 12 and their difference is 4. The difference between the sum and difference is 12 - 4 = 8.