The number 2 retainers holds the snap ring in place, then once you start driving, the splines will lock/bind up and hold...just like a Ford 9”.

The design of the Wilwood kit sucks, but it works OK. Also, either you need to remove the center button/thrust slug from the differential...or, what I did, was have a precision grinding shop take off 1/16” off the end of each axle, still leaving a little chamfer.

But, if you have some free play, you will be ok.

Mark