Sounds like you are talking about a bottom end problem(main or rod), as
opposed to an engine top end problem(valves or lifters). I once knew of
someone who had a 84 or 84 Nissan pick up with this very problem. It was
either still under warranty , or just off warranty(long time ago). The
owner said that the dealer said not to worry.However he still had to live
with it. The dealer never gave him an explanation, though I believe the
dealer knew what the problem was. Just wasnt serious enough to tear an
engine apart.I would have raized hell if it was my vehicle. Keep searching
till you get an answer that makes sence to you, and seems right.Maybe try a
Nissan pickup forum.