This platform doesn't have "old school" drum brake shoes.... the parking brake shoe is a single piece per side. I have had one hell of a time getting mine to even attempt to stop my car. It is entirely possible that the inner diameter of the drum is out of spec... but let's try some other things first.
Before working on the parking brake, there is a little "button" at the bottom of the brake lever that you need to push. This releases any pre-tension on the brake cable. The parking brake cable is self-adjusting and the lever will constantly try to take out slack. Pressing this button will release the pre-tension. This will become important later.
When I change my parking brake shoes I usually knock off the lip that forms on the lip of the drum. Sometimes it takes some grinding with a stone, but usually just hitting it with a scraper/putty knife will do the trick (Wear at least a dust mask if you do any grinding whatsoever). It's usually just rust and comes right off. Then I CLEAN the inside of the drum VERY WELL, including using brake cleaner.... then when reinstalling the drum back on the axle, I usually adjust the star nut ("adjuster" technically) so that the drum barely clears the shoe. If you're installing a brand new shoe after removing a worn or broken one you will likely need to rotate the adjuster to make it smaller instead of larger - regardless, you want the drum to just barely clear the shoe, without dragging. While you have the drum off, if you look behind the adjuster, there should be a knockout you can punch out (if it's not already knocked out). You can actually turn the adjuster with the drum installed if this knockout is removed. I haven't removed mine on my current Bravada, but had to on my wife's. Once the drum is installed (barely clearing the shoe), pull the brake handle, let it set, and release it several times. This should take out any slack in the cable. You may choose to completely reinstall everything before doing this. After everything is reinstalled, and you've pulled the brake handle several times, you an check your parking brake for effectiveness. I usually do this by putting the car in drive, setting the brake, and easing off the brake pedal. If it holds, yahtzee, if not, the adjuster needs some work. Rinse & repeat. Pressing that pretension release button may or may not be necessary on subsequent R&R of the drum.
If you absolutely cannot get the parking brakes to "grab", your drum is very likely out of spec - there may be no good way to get them to grab without replacing the drum. Brakemotive is a decent brand option for replacements.
Some words of caution.... I NEVER jack up a car and remove the wheels without putting the wheels under the frame... NEVER spray brake cleaner on HOT parts, bad gases can form. Safety glasses are always a good idea... always. Whenever you work with one or more of the back wheels removed, CHOCK THE FRONT WHEELS. And there's never a bad time to check your brake fluid except while in motion - that's kinda hard to do.
ALSO... I usually spray out the backing plate (what all the brake hardware mounts to) with brake cleaner. You'll almost never get it "clean" but you want to try and knock as much of the crud off as possible. Proper and judicious use of a good brake-rated synthetic grease on SLIDING brake parts can save you headaches down the road (apply a tiny amount on the adjuster's threads). If you find GEAR OIL brake drum REPLACE the axle seal, or you will regret it.... possibly VERY BADLY. Grease/Oil and brake parts are a bad combination. When in doubt, buy double the brake cleaner you were going to... AND USE IT. I usually buy 1 can per wheel, plus 1 can.