If it were u-joints, it'd make the noise whenever that rear shaft is spinning man... It's not u-joints if it's going away when you pop it in neutral... It's either tranny, t-case or the rear end... Almost sounds like a pinion bearing going out...
43 hours? Kewn, I just finished putting in 114 hours in 2 weeks... Try that one.. Good $$$ but = no life lol