The NV4500 was a 4 speed transmission which was converted to a 5 speed and by doing so, the fifth gear hangs outside the main housing area off the back of the transmission and is isolated in its own area. Not a bad design but the problem is the Cummins is too strong for running it hard in 5th gear because the shaft that the gear sits on is not splinned the entire length of the gear depth and therefor can become a weak area potentially allowing the gear to "rock" back and forth on the shaft splines, eventually pushing the nut out of place and gear coming loose from the shaft enough to disable 5th gear. It can be fixed with updated nuts but generally the problem is that when it happens, it compromises the shaft and gear splines and therefor will continue to happen until something is properly replaced. The best method is the fully splinned shaft and proper nut design.
Its said that in order to lessen the chances of this happening you shouldn't tow heavy in 5th (which is just common sense) and you shouldn't tow in 5th gear with the RPM's less than 1500. But after all the research I've done on this I disagree with the masses and I believe that the problem is driver habits. What I mean is..... People have a habit of driving along in gear and then rapidly letting off the throttle, and then back on throttle. Some people I've driven with in manual transmission vehicles do this to the point where I'm wondering if they have any clue to how harshly they're driving. So what I've come up with in my mind is as long as you dont drive in 5th gear with that horrible ON/OFF type throttle control then you're far less likely to rock the gear in that back and forth movement and therefor it should hang on fine. To reassure my point, do some research on this topic and you'll find more often than not that someone describes their experience in the manner of "I was driving along in 5th gear, let OFF the throttle and then when I went to accelerate there was no 5th gear". That tells me that their driving habits are very ON/OFF with the throttle instead of using a gentler throttle transition.
Aside from that, Google NV4500 5th gear nut fix and you'll find all the information you need to understand what happened.