My breeder has a similar problem with one of her dogs. If I remember correctly, she said she has placed cattle gates (long gates you can get at any farm supply store - like Orschlen's) against the fence and covered them with tarps so the dogs can't get up there and climb - they slide off the tarps. This would make the fence more like 9-10 feet tall at least. Of course, you want to make sure you secure the gates to the fence well so the dogs don't pull it down on top of themselves. Also, my breeder lives in the country - this method may not work as well in town. Sorry I can't be more specific than this, but I hope this at least gives you some ideas.
Ali & Maggie