The nut is a standard part available at any truck dealer. The threads are another matter. Depends on how low they are worn. There should be a lock ring between the nuts. The inboard nut should have a pin sticking out of the outboard face that engages a hole in the lock ring when it is adjusted. The outer nut is tightened securely against the lock ring.
If there is still enough there, you might try a thread repair file:
https://www.mcmaster.com/thread-repair-files Certainly worth a try. You must use the correct pitch file to match the threads, I do not know off hand what that pitch is. You could measure it from the good threads remaining. Keep the file carefully in line with the threads and only file until the teeth reach the bottom of the V of the threads. You can see the bottom start to glow as the file touches the metal in the bottom. Go around the spindle carefully, it will take a long time but you might have some luck.
I don't think the spindle is replaceable, maybe someone else knows for sure.
