I noticed that the gearbox was coated in oil around the spot where CV axle connects to it, which I've come to a conclusion was caused by a failing oil seal. The first problem was draining the oil out of the gearbox as someone (probably some sketchy service shop) has completely gnawed off the specialized triple square socket for the fill plug:
As expected, no repair shop wanted to deal with it, as they only care about making a quick buck on trivial repairs for people who can't be bothered to do simple maintenance on their shitboxes, so luckily my neighbor came to save the day by simply welding a bolt onto the plug and then successfully screwing it out:
...and here's the new, more standard plug:
At this point, I could finally get to the annoying part of the job, which is slowly unbolting the CV axle from the gearbox to get to the oil seal, which, after some help from a wood screw and pliers, was successfully removed:
...after which the new one was gently tapped into place:
Now, at this point I decided that, since I've went that far, I might as well take a look at the condition of the internal CV joints, and it seems that was a good call, as after soaking it in diesel and brushing it off, I found some quite noticeable wear on them. Considering I saw the original VW stamps in the metal, they served their 20+ years of ~500k kilometers well enough:
Now, at this point it probably would've been more wise to just change the entire CV axle entirely, but I already had ordered replacement joints and boots, so I went with that.
And finally, everything was assembled back and was looking (and behaving) better than before:
