Since the previous owner welded up the diff filler plug, refilling it from there was not an option. While sitting underneath the 4Runner today, loathing the previous owner, I noticed another bolt. The vent. TL;DR: We serviced the diff through the vent.
Yes, I filled it up one cup at a time through that little filter... For your time, the Slowrunner repping Oppo, while overlooking a fire.