Abs Problem

99 ranger 4x4 4 wheel ABS........the last 3 or 4 feet of stopping the ABS
starts to pulsate and the truck pulls to the right if i let go of the wheel..
I have been told to replace the wheel speed sensor but i don't know which one to replace.....I have one here and at $100.00 buck i want to replace the bad one only......so do i do the left or the right????