Could someone have possibly swapped the stub shafts? The hub should be fully engaged at all times, there shouldn't be room for the shaft to move that far in so that it misses the splines.
The snap ring on the stub is only there to hold the axle shaft close to the rear dust guard/seal to keep crap out of the spindle. I was sick of killing spindle bearings and getting my lockouts wet, so I installed a UHMV insert that replaces the seal and bearing - even after being 100% submerged multiple times everything was bone dry.
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One of my lockouts is busted and I was stuck, making enough noise that a neighbor investigated
I ended up taking the lockout apart, putting the spring on the other side of the lockout and putting it back together. It basically forces the lockout to be locked all of the time - which really isn't a big deal for this truck. It's the pass side too - so I can still unlock the one side to let the driveshaft freely spin.