Something missing in my 8.75. Axle slop

I have too much slack in axles. Took it apart and that part between axles is missing. Looked at an extra 3.91 I have and see it in there. How do I put the missing piece back in. Needless to say, but this is my first rear to mess with.
1st-pic is of extra 3.91
2nd is of 3.23 out of car.

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