Throw out bearing possibly or Ujoints. The pinion nut should be like 250 Ft Lb torque, Could be what was said: Nut or pinion brg. Try down hill in neutral, coasting, that way ring/pinion and ujoints are being driven by the forward motion, not the engine. See what happens. It's something where the torque of the drivetrain is taking out the "slop" and when relaxed is howling. Ujoints are easy and cheap to replace to see if there is any improvement. Usually you can not tell if ujoints are bad by exam or feel while on truck. You are half way there when you check/tighten Nut
