I did read an article on the NV4500 that said it had problems with 5th gear slipping because the retainer nut would back off over time.
I don't know what years are affected, but I've seen the problem dealt with by simply welding the nut in place to assure it won't back off.
While that seems a disturbing approach the impact is you have to replace that shaft if you ever need to take it apart. And that would not be the end of the world...
However I've heard that with New Venture now defunct parts are getting hard to find. Since we all deal with obsolete parts all the time that may not be a big deal, but I always feel an advantage of updating things like transmissions is access to parts and this transmission may not have that advantage.
