Like J-D said, draw it out with a suction device. No sense pulling the cover if it's not leaking. Unless what you take out looks milky from water contamination (driving through any lakes?), not getting every drop out is not important at all. Get out as much as you can and refill with fresh. Changing fluids regularly will make that rear end live longer than you. On the subject of fluids, flushing fresh brake fluid through the brake system can extend the life of the hydraulics significantly. Ford wants every 2 years, I've always done 3 year intervals and never had to replace a caliper or wheel cylinder since I started that regimen decades ago. It it's been a while, the fluid that comes out will look like root beer.
