JUstamom:
Are you sure its the brake losing fluid? It is common for the rear axle to leak from the wheel bearing seal, so what you are seeing may be gear lube actually leaking from the axle seal (axle tube).
An easy way to check is to check the level of the brake fluid in the master cylinder (under the hood)
I assume this is the 91 Explorer in your signature, with rear drum brakes.
If the brake fluid level is not low then the next step is to check the fill level of the rear differential. If low top it off with 80W/90 gear lube. (or have grease monkey do it)
then plan to have the axle seal replaced.
If the brake fluid level is low then it is likely the wheel cyliner inside the drum brake. This is the little hydraulic actuated cylinder that pushes the brake shoes out into the inside of the drum. The wheel cylinders themselves are pretty cheap (like $12) but they do require taking apart the drum brake assembly. Also if you take the drum brake assembly apart it is good practice to replace the brake shoes, spring kit, and wheel cylinders at the same time, both sides (brake job).
If you have never done drum brake before you will need either a friend who has, or the Chiltons or Haynes manual for your Explorer.
Perhaps a local Explorer forum member can help??
If the brake fluid is LOW when you check it is not the best of ideas to continue driving the truck depending on how much it is leaking.
Also you might jack up the rear axle off the ground and make sure the brake is not seized. With the E brake off and BOTH rear wheels in the air (truck in Neautral) the wheel should spin freely.......
I say you have to jack both wheels off the gtround because the truck may be equipped with a limited slip rear diff.
Hope that helps........