I worke at lark back when these trucks first came out. and this was a common issue with them from the factory. trans and diff would get air in them and then start burping the fluid out while sitting most times over night. the factory fix was. They had marked the dip sticks wrong and to file a mark in the dip stick about a 1/8 to 1/4" below the mark now and use that as your full mark draining the fluid down to the new mark. Or have even removed the vent from the diff. installing a fitting with a hose bringing the vent up higher and securing it somewhere around the main valve. this allows the air to escape without the oil getting that high. seems neater and better than filing a mark in the dip stick and explaining that to the customer. LOL