My rig came with braces that go between the inside wall and the lip around the inside of the slide - up at the top of the slide. Designed to keep the slide from sliding out if something goes wrong. Doing something similar with 2x4s as mentioned makes sense.
I'm thinking a "broken" motor is a brake in itself. Have you tried to push the slide out? Or if you can you get to the bottom where the drive assy is,maybe you could clamp onto the actuating rod with a vice grip.