Also think you use a bit too much force and overtighten the screw. You can feel when screw is completely tightened (spring fully compressed). When you have to use a bit more force, be careful.
Use your fingers as a slipper...
Force needed to fully compress diff spring can not strip nut or screw or round diff nut plastic holder.
Lee has written a guide, maybe you have already read it?:
http://www.trfracing.co.uk/page_1265187551237.html
Also, remember to clean/grease the thrust bearing when you rebuild the diff. If thrust bearing is dirty or worn, diff screw will unscrew itself. So check and make sure that it rotates smoothly.
Use a 53577 TRF Damper Urethane Bushing ot protect thrust bearing against dust. Simply put on top of diff screw in the outdrive:
Just a few thoughts...