Has anyone replaced the vent/breather value on the rear differential? Does it require to drain the fluid first or is it simply remove and replace? Part itself is relatively cheap.

Dealer said mine is seized and will require replacement otherwise the axle seals will start leaking.

 

On most cars they are threaded in on the top of the diff housing, draining the fluid is not required. it's pretty common for the cap on top of the vent to get corroded or plugged up with crap and the best thing to do is replace it. You can put pliers on the cap and try to rotate it free but that wont tell you if the passage into the diff is plugged or not.
I would soak it with penetrating fluid before trying to remove it.

 

Only issue dealer found is that it is corroded but they don't want to replace it because they fear it may break or partly crumble and fall into the differential. They don't want to own that which I don't blame them. New diff is $$$$