Well, just a thought - but this seems to happen on vehicles that are several
years old, not so much when they are new. Which may mean it has something to
do with the diff fluid getting tired and the additives wearing out.
Might be worth replacing the vent tube (old one is now full of crud).
Reroute it upwards and terminate it at the top of the radiator support, where
you can check it easily when you open the hood. Keep the tube running
upwards at all times so it drains back to the diff easily.
Then use a good quality 75-140w gearlube. I doubt the fluid will climb that
high. You might be able to spend more money on Christmas presents that way.
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