Cause there isn't a bearing or such, the splines are long enough to support each other and if there is up and down / left right play your splines may be shot.
If you have the wheel bearing out, just pull the axle out and take a close look at the splines at it and inside the transmission. Also inspect the little retainer ring at the end and the groove it sits in.
If the splines do look very good, still. Than you have to take the transmission apart and see if the parts in there have play.
Attached two pictures:
Transmission RPO: MN4