I am in process of replacing my rear pinion seal in my 9” axle. I removed the stub shaft between the main driveshaft and pinion yoke. While cleaning up the male splines on the main driveshaft, I noticed it’s got some sort of gasket/spacer installed. The gasket sits on the face of the driveshaft and has fingers that fit between each of the splines. I have one of the fingers pulled up in the picture.

Any idea what this is and if it’s critical to reuse?