If you are uncomfortable with the thought of putting the car up on four
jackstands, then just raise the rear end. You don't really need the front
up to fill the differential. Put blocks around the front wheels so that they
cannot move and also place your floor jack under the car somewhere as
a third "safety point". Sometimes I will even place my second set of
jack stands under there somewhere (not touching, but close) to add to
my level of comfort.
I also usually remove the rear wheels and place them under the car stacked
so that if it does fall, it will have to crush the jack and a pair of stacked
wheels before it gets me. It's sort of a belt and two pair of suspenders approach.
I replaced the diff fluid on my 1984 300SD two weeks ago (as part of the
procedure to replace both rear trailing arms) and did exactly what I suggested above. I think the capacity for my car is 1.1qts, and I used Mobil 1 75w-90 gear oil (synthetic) which is excellent and reasonable at about $6 per quart.
1984 300SD 148k