Maxwell
I had the same problem on my vanagon with the cupping on the hub. Turns
out that whenever the nut gets loose the splines on the hub wears down.
Thus the hub and axle has play in it. When ever you back up and go
forward the looseness between the axle and the hub shears the cotter pin
and the bolt becomes loose. This happens no matter how tight you make the
nut. The fix is to get another hub cause this is softer than the hardened
axle and will wear first on the spline. If the new hub still has play
after it is fit on it will have to be replaced also. On mine luckily I
figured it out early and only had to change the hub. Hope this helps.
Aloha, Ed