As for changing the front diff lube, jack up the car, use jack stands
, remove the big hexagonal drain plug in the bottom of the finned differential housing, (I'll see what size when I get home) drain it into a pan, after you re-tighten the drain plug, refill it with 1.3 us quarts of 75/90 gear oil. The cheap stuff is $3.00 a quart, and Mobile 1 is $8.82 a quart, the rear diff takes .8 us quarts. Those drain and fill plugs have a 1/2" socket cast into them. I would go the extra $12.22 to get the good stuff.
Advance Auto Parts has it, pn;FK280 ( with the 7/8" pickup tube) for $25.48, so your auto store should be able to get it, but bring in the old filter to compare/ match up.