Is there an adjustment procedure for those parking brake shoes? If not, how about the center cable adjustment on the Jeep?

Actually I have push started a manual! This jeep in fact. Didn't work in 1st or 2nd but it did in 3rd.

The adjustment is in the cable and I've got it right enough that it's difficult to get it all the way to the top.

Or it sheared. They aren't made of much. Depending on that alone has wrecked many cars.

It's a parking brake. Use it. Every time. And like every other system on your vehicle, keep it in working order!

You have that backwards. A lower gear (1st or reverse) holds it better on a hill since the wheels are trying to turn the engine. In circumstances where the engine is turning the wheels (e.g. driving up a hill), it's more difficult to move the wheels in a high gear. The amount of mechanical advantage depends on which end of the system you're applying force to.

We all know that car companies cheap out on a lot of things. With that in mind, they would not spend all that money putting a parking brake system in there if it wasn't needed.