Backing plates seem fine. I don't see the shoes hanging up on either side on either wheel. The hoses look fine too on both wheels. Everything was put on brand new and ALL components have only abut 6000 mile on them.
So when you say "all components" I assume you mean the springs and pins were replaced as well (which is what I was going to recommend).
The next thing (IMO) is to remove the drums (both) and inspect everything, ensure you don't have the long shoe on the front, springs are where they're supposed to be, shoe pads are clean/dry, etc.
Theoretically you have the same hydraulic pressure to both sides. So that excluded, it has to be something mechanical.
