Joined: Sep 2002
Location: Winnipeg, Manitoba, Canada
In my experience, a steering wheel vibration usually indicates unbalanced wheels/tires or some other loose front end parts (i.e. ball joints, tierod ends, bearings, etc.). However, I would not expect this vibration to go away when in neutral, because the wheels are still rotating more-or-less at the same speed.
You could check the wheel balance thing by simply swapping the fronts to the back and taking a test drive. If the vibration moves from the steering wheel into the seat of your pants, I'd say you wheels/tires are at least part of your problem.
One other thing....
You mention that the vehicle has "some rust on the undercarriage". I suggest that you check the right and left frame rails at the back of the vehicle - specifically where the frame rails "hump" up and over the rear axle tube. Early 90's Pathfinders are notorious for these frame rails rusting, especially in this location. Last spring I found that the outer side of both frame rails on our '92 were completed rusted away for about 2 feet, right over this "hump". I didn't see it until I removed what was left of the rust coating on the frame. What I found scared the crap out of me. I had it repaired at a body shop for almost $2,000 (Canadian). The body shop confirmed that this is a very common Pathfinder ailment, but that mine was the worst they had ever seen. I don't mean to panic you, just giving you a head's-up. I strongly suggest you take a look at yours.
1992 Pathfinder V6-SE 4wd