As already said above make sure the spring is free to slide in the outdrive. I use a dremel to go round the edge of the spring a little to make sure it doesn't foul. What tends to happen is the spring catches so doesn't compress properly, then the plastic nut holder is put under too much stress, this causes either the nut holder to break, the nut to strip the nyloc or the nut to spin round in the holder. All of the above cacks the diff and it doesn't work properly.
Follow the Lee Martin guide to building diffs and dremel the spring for a better fit and you'll have perfect diffs (ceramic balls are a must have)
