I saw some swaps on youtube with the vw 1.6 TD engine in toyotas but no nissans, though I've looked extensively. my truck is powered by the 2.7TD it was born with, I'm happy with it for now and for us here in Eastern Europe it's a very common engine, easily found in WD21, R20, R50. I tought the newer Pathfinder/Terrano models came in your markets with diesels too -thus making the swap a bolt on.
coming back to your question: I'd look for an old Audi, with the VW 1.6TD and a manual gearbox, that you can mount on your chassis and connect to some transfer case (from Suzuki or Mitsubishi maybe?), this way you get the gearbox final drive set for on-road driving and the transfer case reductions for off-road. what's the ratio you have in your diffs now? My standard 2.7 came with 4.625, with the solid H233b in the back.