They nissan aftermarket has really taken off the last 3-4 years with the introduction of the Xterra and Frontier. While that means they hold most of the compaines interest it did bring some upgrades along with it for the D21 and pathfinders.
First thing I would do if you want to run bigger tires or even take it off road would be a steering upgrade. The stock centerlink is very weak and you will bend tie-rods like there is no tomorrow.
Here are some places to get you started.
- This is the biggest Nissan only parts distrubtor out there. Friendly folks Mike and Jane are.
This is Calmini's site, very nice products.
Spencer Low Racing....for all your desert racing stuff. Some very wild IFS suspension setups.
Those are the three big ones I can think of off the top of my head.