Just think about this... Nearly everbody that is running a Jeep/ect off-road is running a NV-4500, if they have a manual tranny.
Just like GMCBLU said, the retaining nut is just a flaw in the design.
Everywhere I read, people say to stick an extra quart of lube in it. The Dodge board
TDR has a few threads about the NV4500, and they say to run it a quart over... AND a lot of the guys on that board and turning some big torque #'s!