Most times when someone has a leaking rear hub it's the inner oil seal. If you have a leak at the axle gasket, then all you need to do is take off all those nuts. slide the axle out, clean off the goop and old gasket, and put it back together. A factory gasket would be best but I would probably use "the right stuff" from permatex/loctite. You don't have to drain anything, or take anything else off. That is not a cover plate, btw, that is the axle. It's about 3 feet long, has splines on the end that you have to stick back into the differential, and it probably weighs a hundred pounds or more. Easy job for a truck shop, btw, and if you ever get towed, the tow truck driver will pull out an axle and put on a cover plate so the transmission doesn't spin while you are getting towed.
Edit: it actually looks pretty clean right around where the oil would be if it was the axle gasket. I might suggest getting a look at the back side of the wheels. If it is a hub, then it's a bigger job. It's not an super difficult job, it's just that everything is extremely heavy and the torque numbers are extremely high.
brian