Jittering rear brakes

Greetings all

I'm driving a 1992 Ford Aerostar with drum, antilock brakes on the rear wheels.

And a few months ago I pulled into a parking lot, set the parking brake, and went into a store. Then, when I was done inside the store, I drove out of the parking lot but forgot to release the emergency brake.

And by accident I drove it this way for about 100 miles.

The rear brakes, including the antilock part, still work, but now they jitter when applied.

What happened and how do I fix it?