I have the 11.75 front/11.00 rear (Jeep disc) on my Dart with the stock prop valve and not once has the rear end locked up, even when I really need to stop. Brakes have been on close to a year now.
You should be able to lock them up under extreme braking. Ideally of course all 4 would lock up at the same time, and only when you plant the pedal on the floor.
If you can't lock them up even under extreme braking, then you're not getting everything you should out of your brakes. If its just the rears, then your prop valve may actually be too biased toward the front. If you can't lock any of them, then you've got a system wide issue- air in the lines, contaminated brake fluid, problem with the master cylinder, etc.