Sierra 1500,Overflow tube in the front differential is leaking oil constantly on a 2014 GMC Sierra 1500.

I've been having an issue with my front differential, in the beginning the dealership changed the seals in both differentials as they did not know where the leak was coming form. after returning several time after the fix with the same issue they found that the old was coming form the overflow, they do not know how to fix it is seems from here, any suggesting in the cause and solutions?

My car has 120000 miles.
My car has an automatic transmission.