I busted my dana 80 spider gears too, a few months ago. I think the failure started with wheelhopping leaving the line at the dragstrip. About a week later, my speedometer dropped to zero and stayed there. Turns out, spider gear pieces were circulating in the lube, and some got caught between the carrier and the tone ring that excites the vehicle speed sensor. Not knowing this, I kept driving it. About a week after that, it started binding and popping going around turns. That's when I brought it home and pulled the cover off. All through this, the original stock nv4500 held fine-it was only the dana 80 that let go. Luckily, I had another dana 80 sitting around that I put in.