What's up Mike... The stamped metal cover on the back side should be permanent, as these are sealed/no-maintenance items. Hubs are the same for 2 & 4wd, for a 4wd the axles go through the center, with yes an axle nut and cotter pin. I'm assuming the 'black membrane' is just there to cover the hole if the hubs will be used on a 2wd, otherwise you probably pop them out for the axles. Hope this helps!
2006 Frontier Nismo CC 4x4 AT
* Build Thread *
- BULLY DOG Tuned
- Airaid QuickFit / Custom Y / Flowmaster 'Original' 40
- PRG 2.5" Front Spacers / AC Adjustable Shackles / 2" BL
- WeatherTech Mats & Vent Visors / MTX Thunderform with Kicker shallow CVT
- MetroRestyling DIY Diamond Mesh Grille / Motorsport Tech Adapters / Titan 18x8's / Falken 265-70-18's