Originally Posted by AM03GT

fully tightened the gear should be pretty much locked... the force i use to test this is to hold the outdrives in place, pinch the gear with thumb and pointer fingers and try to "spin" the gear...

if your diff slips with a fully tightened screw you may have a stripped diff nut/t-nut... the diff rings have been polished so they are too smooth and there isnt enough friction... diff balls are worn (unlikely)...