He could lose his axle without the s-spring. If you look at the way it is designed as you corner and your axle moves inwards without the s-spring which kinda acts like a guard around the c-clip so that it doesn't fall out. Without that gaurd I do believe that the potential is there for the c-clip to either come fling out (because of centrifugal force) or just fall out if the axle was pushed in when the car came to rest. Just my thoughts on it when I look at the way the spring is designed in conjunction with a c-clip.
But, there's only one pin?
It's been a while. But, I thought that the pin keeps one axle in the differential and the spring keeps the *other* axle pushed in (actually, it pushes the axle outwards towards the fender). And, with the axle pushed in, the c-clip was in a recess that prevented it from coming off. Again, it has been a while!
Remembering things with the differential are very hazy. That TOXIC Ford limited slip additive still causes me to have horrible nightmare flash backs.
Its only an axle, Ford saw fit to bless him with a spare... sorry stole that from 300.
Kevin