This has nothing to do with your question, but the rear end on my wife's 2000 F350 went out at about 100,000 miles. She pulls a heavy trailer, but I don't think she stresses the truck beyond what it is supposed to handle. The truck has been maintained by the book (better than the book, actually). It did not have a seal problem. The differential just wore out, which was disappointing. It is the only problem we've had with the truck beyond the usually stuff. Cost about $2,500 to fix.