Hi Richard,
The only thing I would say here is that as long as the L2 switches have correct default gateway all packets comes to gateway and from then gateway re-rute the packets to any other destination.
As you are saying they are L2's and still you are able to reach onsite (I understood other remote location) from L2 switches but not backbone.
Where is your backbone and what is it also where is the onsite?
Was there any change in the physical connection in your network?
Please rate the helpfull posts.
Regards,
Naidu.