Brakes grab at very slow speeds

I'm a little confused. When you say total lockup at low speeds when backing out of a parking spot, how did you get there if they're locked up? Part of what you're describing sounds like they are just dragging and not fully releasing. Locked-up brakes won't get hot either because the wheels aren't turning. Dragging brakes will get real hot. That is a sign of trapped brake fluid that can't release back to the master cylinder. The two most common causes of that are a restricted rubber flex hose and brake fluid that is contaminated with a petroleum product.

I think you have already identified one way to tell the brakes are dragging. Another way is to stop on a slight incline, shift to neutral, release the brakes and see if the vehicle creeps downhill on its own. When it does not, put a block a few inches downhill of a tire, (so you don't have to go running after the truck!), Then loosen the steel lines at the master cylinder. Lets hope the brakes do not release there. If they do, the brake fluid is the problem and that will be a very expensive repair. A misadjusted brake light switch can cause that too. We'll discuss that if it comes to that. With contaminated brake fluid, the additional clue is the rubber bladder seal under the reservoir cap will be blown up and mushy.

The next thing is to open the bleeder screws on the calipers. If that lets the brakes release, suspect constricted hoses. It is not common for two to cause problems at the same time. Normally only one acts up and just that one brake gets hot.