This is as much a plea to Durango as much as anything. I know that this is not a universal problem, but I have suffered constant problems with the ball diff slipping, so much so that I have replaced it with an associated diff, which has cured the problem. The problem (highlighted to me by a friend from racing) as far as I can see lies in the thrust balls and plates not able to have enough pressure exerted on them, which is due to the thrust being on the inside of the outdrive rather than the outside. So if anyone has had the same problem please reply to this tread to highlight the problem, and do demonstrate it it not just me that cannot build a diff properly!
Maybe if enough people reply something will be done about the issue, and if not then I will carry on using my associated diff.