Forgive me, but is this thread a joke?
Women are not ignorant or incapable of learning, and if she indeed posses some mental deficiency or stubbornness that prohibits her from learning the 4x4 system in a Frontier, which is pretty stupidly simple, why would she be driving it, on or off road?
If that is her idea, it sounds like an excuse because she doesn't want to drive it. If that is your idea, you sound like the most chauvinistic man I've ever heard of. I wouldn't even entertain the idea of rebuilding a vehicle's drive-line with a hodge podge of parts to create a center diff 4x4 just because she was unwilling to learn how to turn a knob, stop, put it in neutral, hold the brake, and turn it again.
The Frontier 4x4 system as mentioned is very solid. I've driven them for years in all manner of conditions. It can be a bit finicky if you try to do things out of sequence, but it's by no means difficult. RTFM. It is pretty inexcusable to drive a vehicle and not have the ability to learn it's user interface. I mean it's not like they have locking hubs and you have to back up or get out and lock them, we are talking about a 4x4 system you can completely manipulate from the drivers seat.
If that is really what this is all about, leave your wife out of it and get the vehicle that suits your needs and wants. Don't imply she is too stupid to drive a Nissan.