If the rear diff cover looks like this except it has the rubber plug like you mentioned - yep, it's a D35. This cover is for a D35 too, but just an earlier model non c-clip D35.
As far as the plug goes....they tend to get brittle and easily crack and tear over time. Be carefull removing it because you can easily tear it & then it'll start leaking fluid and be a mess.
If you do decide to change the fluid, simply remove the whole cover carefully so as to not give yourself an oil spill, then remove the plug from the inside pushing it gently out. Clean the mounting surface of the diff & cover. Inspect the diff internals paying close attention to the gear wear and potential chips in the spider, side, and ring & pinion gears and also clean all the fine metal shavings out from the bottom. Apply some RTV gasket sealer to the cover & mounting surface. Reattach the cover. Refill with appropriate grade oil, then put the plug back in.
I also like to spin the diff while it's opened up to further inspect things.