Because I've just been though this myself, I thought I'd add my clever solution to this problem.
In the midst of doing a rack and pinion R&R, I realized I never got around to changing (or even checking) the front diff fluid when I bought the vehicle in 2012 with 170k kilometers on it. I've driven it 6 years and additional 200k km since then!
So while it was up on jack stands I figured this would be the perfect time to get it done and changed over to synthetic. Drain plug was quite easy, probably due to the plastic washer and whatever fluid remained in the diff (there wasn't very much and it was black as hell). However there was NO WAY that fill plug would budge - not after torch, not with impact, not with 4 feet of leverage added to my 2 foot breaker bar (and 6 point socket). I was actually scared I was going to break the case.
So, I took 6 inches of rubber brake line, jammed one end into the end of my fluid pump hose and inserted the other end into the differential drain hole. The brake line was snug enough to keep the fluid from leaking out as I slowly pumped a measured quantity of fluid into the differential, plus a touch more for the anticipated loss when I replaced the brake hose with the drain plug.
I figure if the (very little remaining) OE fluid lasted me this long, any new synthetic fluid at the proper level will be good for the rest of the vehicle life. Hopefully this solves a problem for others in the same boat.