I read in another thread about some device that just looks like a birdhouse, but which puts out a high-pitch sound that only dogs hear. It's guaranteed to stop them from barking, and it works up to 50 feet from the device. I'd put that sucker out in your back yard, and just see what happens. I think they were around $35 and were found online at something like Dr's Foster and Smith or something? I hope someone else will have more details, but that sounds like the best solution.