Ford E-350 Rear Brake Squeal

Would anybody know what would cause my rear brakes to squeal on my 1993 Ford
Econoline -350. I had the brakes replaced and I get a squeal on one rear
brake. I had ford check it and they removed the shoes and chamfered the
ends. This didn't help anything but a sometimes lockup on the same wheel.
They said to live with it for awhile and see if the squeal disappears. It
has been approx.. 30 days and many miles and I still get the annoying noise
when I brake at a light or stop sign. Has anyone had the same problem out
there or could you suggest what can be done to stop this.

Doug -