It's somewhat "normal" wear..... the side yokes are pushed against the center shaft causing them to wear, some replacement yokes are not hardened at the tip....
TT is right on with this, if you have axles without a groove then approx .200" has worn away and you most likely need an overhaul. There should be a /18" lip at the seal, if it's gone it already has hit the diff.
Now if you have aftermarket axles, they "should" have the groove and I haven't seen one without it but if there is no axle endplay who knows.
Most likely you have worn axles- although original 63-72's are pretty good and many I reuse. 70 diff's are kind of rare in respect to the numbers because of the strike that year. They were the only ones with the 3 letter prefix stamped.