Left rear axle popping out of differential

[Blacky]

I finally figured out why my AWD is not working. The left axle pops out of the diff slightly and bingo no rear drive. When the axle is popped back in the rear drive works.
What is supposed to hold the axle in place? Is there a circlip or snap ring which has come off inside the differential or is the end of the axle worn and needs to be replaced?
I have a line on some used rear axles but don't want to buy them if I only need a circlip or snap ring.

Thanks and Merry Christmas to all!