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I just had a look on my truck and I see the similar area where the vent tube should be , but on mine the tube continues on towards the front of the truck, I thought there would be an opened end.
I found the vent, the hose was not plugged in, therefore the vent nozzle on the axle was completely blocked. I couldn't unscrew the nut, so i stuck a piece of cut coat hanger down the hole to loosen up and sprayed some compressed air to shoot out the gunk. Eventually i free'd up the crap and blew more air through to let the diff build up some pressure, let go and air pressure blew back out on its own. Thanks for every ones help, i kept blowing a seal due to the pressure build up to my clogged vent.