edit: fixed I think the fill bolt was slightly loose.

Could be drain/fill plug gaskets, pumpkin gasket (where 3rd member assembly connects to diff. housing), or pinion flange gasket (where drive shaft/pinion flange connects to 3rd member housing).

I see nothing oily in front of the pumpkin, so unlikely it has to do with 3rd member oil stan is between the 2 drain plugs.

So it looks like oil change is in order.
Do dealers still have fill and drain plug gasket these days? If I remember they are a metal gasket? (edit: seems part # is 12157-10010 )

80W-90 sounds right?

If the main seal is leaking, you'll likely have a big oily mess under the car as the oil gets flung onto the body and everywhere else. You should see a drop hanging down from wherever it is coming out of. If you put extra oil in the rearend to help keep it cool, you could have overflow coming out of the vent on top of the rearend too. I've noticed a few drops trickling down the housing from the vent after a track event.

It looks like I need a good degreasing and observe, I didn't know there is a vent on top

That little aluminum thing on top that wobbles around is the vent. It might be covered in dirt and grease.

How hard is it to replace the pumpkin to 3rd member housing connection.
Is it sealed via sealant?

It's not really hard to do, but a little time consuming. You'll want to rent a slide hammer with wheel stud attachment, and a brake service tool set from you local auto parts store since the axles need to be pulled.

The type of gasket depends on if it's been serviced - most people use silicone gasket maker. OEM is just a paper gasket though. Just clean whatever is there with a razor blade, brake cleaner and rag. If you decide to use a paper gasket on the 3rd member go with felpro. It fits better than the oem gasket and it's more durable.

And when fitting the big round differential gasket fit it to the differential carrier rather than trying to fit it over the studs on the axle.

I'll add that you need to make sure to scrape off all of the old gasket. It's difficult to get it all when the rearend is on the car, so take your time. I'd use an Xacto knife with a scraping blade or similar to get around the studs. Once the gasket material is all gone, be sure to degrease both mating surfaces and be careful not to damage the paper gasket. The final thing to do is torque all of the nuts to spec. If you do all of that, your rearend won't leak from there.