All drum brakes in use on American (and probably foreign) vehicles since about the mid-1930s have been and are what's called a "servo-assisted" design. The configuration of the shoes, springs and attachments is such that when the brakes are applied while the vehicle is rolling forward, the geometry uses the friction of the shoes against the drum to wedge the shoes more tightly, camming them outward to increase the braking force with less foot pressure required. This is why the shoes have unequal length lining material on them, and why it's important to put the short shoe in the correct position.
The servo-assist is not effective in reverse. Parking brakes are always adjusted to hold the vehicle against forward motion, and if you adjust it just to the point where it holds on a slight downgrade when going forward, I'll guarantee that it won't hold when facing backward on the same slope.
This is inherent to the design of the system. It's not an XJ thing, it's not a Jeep thing, it's a drum brake thing.
