Lost 5th gear today for the second time in a year and a half or so. Is there another shaft I can put in there to replace the one the 5th gear nut goes on? If not aftermarket, does anyone know the stock part # or the name of the stock part?

 

Talk to the guys at Standard Transmission. They have been doing NV repairs for years. look up their products page...
http://www.standardtransmission.com/

 

http://www.quad4x4.com/NV4500%20Rebuild%20Kits.htm

 

Some guys have had luck adding a couple of tack welds to 5th gear. Don't weld the whole thing cause you will never get it off, but a few tack welds seems to do the trick.

 

That happened on my pulling truck, I just put a locking jam nut on it an havent had any problems at all.

 

Sometimes the upgraded locking nuts come loose. That's why some guys have gone to a few tack welds.