If it's only leaking at the axle flange, not out of the brake drum, you've found the problem. Clean everything up with a solvent like spray brake cleaner or carb cleaner, reapply some silicone gasket sealer (just a small bead) and reassemble it. Those axle bolts need about 65 ft./lb. of torque or so, so don't be timid. You don't need to pack the hub with grease; it's lubed by the rear end oil. Just keep the oil level in the center housing level with the fill hole, so a little can flow down the axle tubes and lube the hubs.
If it's leaking out of the bottom of the inside of the brake drum, ignore all of the above and tear down and replace the brakes, and change the inner grease seal on the hub while you're in there.
Jerry
