I have a 1997 Pathfinder which I bought new. There's a vibration problem that feels like it starts in the front of the vehicle. I've had the wheels balanced but it didn't help. The main problem is that it's inconsistent and happens randomly. The dealer refuses to do anything else because it didn't happen when they took it for a test drive.
It's always a tough when "it only does it sometimes". Here are a few suggestions. First, make sure the truck rolls freely. A caliper or rear brake shoes hanging up will give you a nasty vibration once heat builds up enough to warp the rotors or drums. Also, make sure you're not taking the same route every time this vibration comes about. Perhaps you're hitting a portion of road with a funky surface, and blaming the tremors on your Pathfinder. One more thing, when the vibration does occur, put the transmission in neutral. If this stops the problem, we can then eliminate the tires, wheels or hubs as possible causes and look more closely at the drivetrain.
An intermittent misfire in the engine is also a possibility. It could be a spark plug, plug wire, injector or something like that. Believe it or not, a misfire can be mistaken for a wheel balance or axle type vibration when transmitted through the drivetrain.