I just went through this last year with my Al-Ko brakes (4400lbs mind you). it turns out my old controller was feeding a small amount of power to the magnets and they were dragging.
Eventually, after a lot of replaced parts (complete back assemblies), the drums were worn down to the stud!
Check your drums. I found them worn down. if they are worn down, you're looking at between 125-150 per drum to replace them.
If you're like me, you can order new axles, drop shipped near you, and install them faster than doing the drums and backing plates...
just my two cents...
josh