Yup my Preview Render settings were all the same as yours except I didn't have "Remove backfacing polys" checked. I do now and it didn't make a difference either. I've opened Poser 11, let the default scene load (Andy2), I turned off lights 1, 3 & 4. Light 2 defaults intensity is 3,000, I changed it from an Area Light to a Spot Light, turned on Ground Shadows (Display > Ground Shadows). We'd expect to see a shadow going from left to right. No shadows in preview window or with preview rendering.

And here's the results from the Firefly rendering:

Thanks for testing and troubleshooting! I'm stumped as well.