Nah mate, can't be done. The unique design of the rear end makes it a massive PITA. And the kinds of brackets you'd need to fab for it to work makes it a pretty unfeasible project, not to mention extending brake lines and diff breathers and handbrake cables. Urgh. The DIY engineer in me cringes at the thought...
i did it to my 2wd hilux, used a 4wd housing and fitted the 2wd axles back in it. if you had a 2wd navara it is worth doing if you have a long arm kit in the front. If you dont there is no way in the world you will get the thing to sit level as a spring over gives you about 7 inch lift.