Oh the nv4500 has a nasty habit of tossing the 5th gear off - don't worry it won't fall off it's captured on the main shaft. It's a pretty easy fix. I have my own thoughts on why it fails or what causes it to fail...
The gear in OEM style is a slip fit which allows it to rock a little on the splines this vibration will and does work the nut holding loose. You can install the updated nut with the set screws on the side but that's just a bandaide while it will get you back on the road but for how long because the rocking will loosen that as well - trust me I know... One thing I found that helps it along is using the cruise control unloaded in a truck that has it's power cranked up because the cruise is designed with a stock engine in mind so it thinks it has to move the pedal X distance to gain 1 mph but in a modified truck it doesn't have to move it that far.... so what happens is it moves the pedal X distance but then the speed passes right on pass the speed set point and then it gets off the throttle and slows down but then it does it again - you end up with surging which is loading and unloading the gear causing it to rock even more.... I wanted to put in a needle valve in the vacuum line for the cruise but never got around to it to stop this and make the cruise react slower. This will only lessen the chance of the gear coming loose....
To kill it dead in it's tracks you have to replace the mainshaft and gear assembly with a set that is designed with the 5th gear to be pressed on.
http://quad4x4.com/NV4500 5th Gear Failures and Solutions.htm
has just that....
I didn't do this because of the cost. What I did was I replaced the gear and mainshaft with another slip fit gear and double nutted it - two nuts jammed tight with both of them having the set screws and had good luck but I also wasn't using the cruise control unless I was loaded. So I can't say for sure that my fix would work. I did try welding the nut at one time but all that did was cause me to kill the mainshaft and it broke in two pieces on me leaving me stranded on the side of the road during a move.... not cool.... So if you choose to weld it don't weld the holy snot out of it like I did... I was a little under the weather at the time I welded it (12 pack in me) and naturally when my nephew said you can't weld that I bellied up to the challenge.... the weld the next morning did not look too pretty - actually it was nasty.... I burned the snot out of the shaft and so naturally it failed.....
