Have you replaced the flex lines to the wheel cylinders?  Over time they can/will collapse on the inside while looking normal on the outside.  The collapsed interior can create all sorts of problems.

I have a weird rear brake problem that I haven't been able to suss out. The brakes feel and act as though the drums are warped. I've replaced the shoes, wheel cylinders, adjusting screws, adjusting wedges and the drums. Also have Timken bearings installed in the hubs. In chasing this down I had the drums machined; it made no difference. Starting out cold the brakes all around perform normally. After a while when everything is warmed up, the rear brake grabbing begins. Additionally, they make noise that would be like a disc brake squeal, but is a lower note and more harmonic and only just as the car comes to a stop.

So, what do you think it could be?