1976 Ford Mustang Rear differential

My friend has this old clunker and it really runs OK, but two nights ago it stopped pullling. I got out and the rear right tire was sticking out about a half foot. The entire assembly, the tire and hub and spindle had come out of the axle. We raised the car and simply pushed the thing back in and it pulled fine until it came out again. What is it in the differential that holds the spindle in place? I suspect I will have to take the rear plate off the differential and replace some kind of retaining bolt, but can you give me some idea why the entire assembly, that is, the wheel, hub and spindle would all slide out?

Thank you.