WOW, That is too bad as it is a LOT of work to change an axle.
First I would try to "Restore" as much as the thread as possible. I have an old style thread chaser that is made like a big clamp that can be clamped over any remaining "Good" thread you may still have. You rotate it back an forth and slowly tighten it every so often. I have gotten bad threads back to the point where along with a new, good quality nut, have had success.
After you have done the best you can do to "restore" as much thread as possible, I would get a new GOOD OUALITY HARDENED NUT and face off the side that contacts the washer so as to get rid of the internal chamfer to expose a sharp theread right out to the edge.
I would then look for a little thinner but hardened washer. This may get you one more undamaged thread.
I would pull the drum and carefully see if you can face off the end of the hub just a little to expose a little more "good" Thread on the axle. Just be sure when everything is tightened up, you will not bottom the washer at the end of the threaded part of the axle and you are actually tightening the hub.
I would also remove the axle key and lap the taper in the hub to the axle taper. This will allow less torque to keep the hub from working loose on the axle. Be sure to fit a new key if necessary. You want the key to fit tight in both the axle and hub.
Then, clean everything real good with acetone, MEK or Lacquor Thinner and use High Strength Stud and Bearing Mount Lock Tight and put everything back together. I would only torque the axle nut to about 100 Ft Lbs, even though the book says a lot more. The lapped tapers and lock tight will keep the hub from working on the axle. Let everything set overnight.
Check several times after driving the car and see if everything stays tight.
This may sound like a lot to do but it is actually quite simple and MAY keep you from having to R&R the axle.
Good Luck and let us know how you make out.
Chris