Unless you've downsized the wheels and tires that came on your 4RNR, you shouldn't need to use ramps or a lift. Park your truck on a reasonably level spot and slide/pull yourself underneath. You will probably need to move the truck forward or back a few inches to rotate the driveshaft so the zerk fittings are accessible underneath from a side angle.
Using ramps might allow you to roll under the truck on a "creeper," which would keep you off the ground and keep you from getting quite as dirty as you might otherwise. If you have access to a lift that's the easiest (and cleanest!) way to grease the driveline fittings, but most of us aren't that lucky.
