The rear end breather is as you describe, mine255
, and dirt get in and plugs it up along with hypoid oil vapors that condense. When you find the vent opening wash it out with solvent but be careful not to get wet solvent into the vent as it can dilute hypoid oil.
At the worst, dump the hypoid oil, it may be entrained with tiny bubbles of air. Let the draining continue as you take a coffee break. Top up the pumpkin with 80 or 80-90 wt oil, if you have LSD then use LSD hypoid oil before you put in new oil, before and after you wash out the vent, if you have a compressor blow low pressure air into the vent and listen near the diff for air rush. This verifies the vent is open. Would be a good time to check rear brakes and bearings and seals for hypoid oil leaks.
As a PM and since hypoid oil is cheap I change it out every other year. Helps clean fragments if any from wear and tear and at the same time check venting, if you have vents.
Save the old hypoid oil, let sit undisturbed, then when done working on the car run you finger to the bottom of that old oil container and feel for fragments. Lts of dusting then change you diff more often, no fragments to see or feel, then keep changing as you are doing.
I am sure you will have dusting of brass, steel particles with the darkened oil. The hypoid oil darkens with age and contaminants and if the older oil is darker than the new it's not a waste of your time. Make it a regular PM.