I know this may sound obvious but when you last stripped your diffs, could you possibly have put the diff back in the casing the wrong way round? ie, with the bevel gear on the left hand side as you look at it from the front. For the diff to fail so catastrophically, it must have been something REALLY wrong with the setup. If you had put the diff back incorrectly, you would have essentially been running the two diffs against each other until one of them gave out. Just an idea.