I know this topic has been beat to death. Sorry! 
I just bought a 2001 4 x 4 five speed with the NV 4500. At this time there has been no failure. My thinking is open up the back of the transmission verify torque and weld the nut to get some more life before an ultimate rebuild. Maybe that would be a waste of time and effort? 
TIA

 The problem with welding is that will complicate things later and still may not hold. The core issue is the oem main shaft is not fully splined for the 5th gear. It causes the gear to wobble on the splined shaft which in turn works the nut loose. Even welded, with the torque the engine pushes through the trans will eventually break the welds ad well.

 I would run it til you have to rebuild it and do it right, replace the main shaft with a full spline piece, use a 5th locking system and loctite.

2 minutes ago, Doubletrouble said:

 The problem with welding is that will complicate things later and still may not hold. The core issue is the oem main shaft is not fully splined for the 5th gear. It causes the gear to wobble on the splined shaft which in turn works the nut loose. Even welded, with the torque the engine pushes through the trans will eventually break the welds ad well.

 I would run it til you have to rebuild it and do it right, replace the main shaft with a full spline piece, use a 5th locking system and loctite.

The only thing I see it complicating later after a failure is taking it apart. The shaft is junk from day one, so when it comes time to take it apart, cut it in half with a cut off wheel remove the damaged section to complete disassembly. Am I missing something?

 You would have to cut through the gear itself and the shaft. The gear when installed is very close to the case so getting in between the gear and case to cut the shaft will be nearly impossible.

I’ve never physically been inside of one these, so bare with me please. If there is a failure won’t the gear and nut be out of there original position? This should allow plenty of shaft access for a cut off wheel? 
Thanks

 True, I was thinking of my situation.  I to was considering welding mine a year ago or so as a precaution, I didn't. Fast forward to late October '23 and the trans grenades. It wasn't the 5rh gear, the front counter shaft bearing ate itself. 

 Had I welded the 5th gear I would have had to do what ever it take to cut through, remove, the main shaft.

 I also like to know when something is done correctly rather than wonder if a bandaid I put on something will hold.

 In the end, it's your truck, do it however you want. I'm just offering my opinion based on some of my experience.

1 hour ago, Doubletrouble said:

 True, I was thinking of my situation.  I to was considering welding mine a year ago or so as a precaution, I didn't. Fast forward to late October '23 and the trans grenades. It wasn't the 5rh gear, the front counter shaft bearing ate itself. 

 Had I welded the 5th gear I would have had to do what ever it take to cut through, remove, the main shaft.

 I also like to know when something is done correctly rather than wonder if a bandaid I put on something will hold.

 In the end, it's your truck, do it however you want. I'm just offering my opinion based on some of my experience.

I value your opinion and I appreciate your time. Hope you don’t mind if I continue to pick your mind for a bit.  Had you done anything with 5th gear before it grenaded? How many miles on the tranny?  Is the countershaft bearing another weak spot on these transmissions?  Have you looked at a 5600 conversion? Did you rebuild your 4500?
Believe it or not  I am of the same opinion about knowing something was done right. I just hate throwing good money after bad. 

I want to start using this pickup to run back and forth to AZ and tow a medium sized fifth wheel and I  abhor the idea of slinging fifth gear and running 1000 miles or better home in 4th gear. 

 I did not do anything with 5th but the previous owner did. It had a non oem nut on it. It was a split nut with a small bolt to tighten it up on the threaded shaft. When I did the rebuild it came off with little effort. I belive it would have failed eventually. 

 Tranny had 183k when the bearings failed.

 The bearing aren't considered a weak point. I belive the bearing failure is due to the thin oil used per oem and possibly lack of proper oil change intervals. When it failed I had put about 45k on the truck so I blame both the PO for not changing the oil and also myself for not doing it when I first purchased the truck.

 As far as swapping to a 5600, there really is no gain in my opinion. The final ratio is very close to that of the 4500 and parts availability is an issue also parts for the 5600 are much more expensive. The 5600's weren't as widely used as the 4500. 4500's could be found in dodge, GM and ford trucks along with the likes of panel trucks such as FedEx or ups use. If those trucks were a diesel and manual trans it was essentially the same engine/trans that we have in our truck. (The big panel trucks, I think they're like a 2.5 ton platform)

 A friend of mine at the moment is rebuilding his 5600, it going to cost him close to 3k just in parts. 

 I did my 4500 myself with new upgraded clutch for about 2k. The clutch was half of the cost.

 I posted a thread on the rebuild, find that and you'll see what all I did. And I listed all the costs at the end as well.

8 minutes ago, Doubletrouble said:

 I did not do anything with 5th but the previous owner did. It had a non oem nut on it. It was a split nut with a small bolt to tighten it up on the threaded shaft. When I did the rebuild it came off with little effort. I belive it would have failed eventually. 

 Tranny had 183k when the bearings failed.

 The bearing aren't considered a weak point. I belive the bearing failure is due to the thin oil used per oem and possibly lack of proper oil change intervals. When it failed I had put about 45k on the truck so I blame both the PO for not changing the oil and also myself for not doing it when I first purchased the truck.

 As far as swapping to a 5600, there really is no gain in my opinion. The final ratio is very close to that of the 4500 and parts availability is an issue also parts for the 5600 are much more expensive. The 5600's weren't as widely used as the 4500. 4500's could be found in dodge, GM and ford trucks along with the likes of panel trucks such as FedEx or ups use. If those trucks were a diesel and manual trans it was essentially the same engine/trans that we have in our truck. (The big panel trucks, I think they're like a 2.5 ton platform)

 A friend of mine at the moment is rebuilding his 5600, it going to cost him close to 3k just in parts. 

 I did my 4500 myself with new upgraded clutch for about 2k. The clutch was half of the cost.

 I posted a thread on the rebuild, find that and you'll see what all I did. And I listed all the costs at the end as well.

Thank you. I’ll look for your thread

https://mopar1973man.com/topic/19825-nv450-5speed-blew-at-184k-miles/?page=2

hopefully the link will work. That'll take you to it.