Over 30 years and 8 or 10 times around the world, what was once tight slowly loosens up, by dribs and drabs.

The nut likely remained properly staked, while the components in between each lost just a little thickness.

 

OK cool, thanks guys for the #'s. Having a similar deal on the 4Runner, recycled the thirds from the 81 and after a month the rear pinion nut appears to be a little loose so hoping this will cure it. Fortunately the thirds were build using solid sleeves so should be ok just tightening
ordered a tripled drill flange, new seal and pinion nut that got delivered on friday

 

A good degreasing of the flange and pinion splines along with an application of loc-tite to those splines can't hurt.

 

good point thanks!