Sometime late on a cool night light off some gopher smoke bombs like these under your shed:
http://www.domyownpestcontrol.com/giant-destroyer-smoke-bombgasser-p-837.html
It'll kill them.
Thanks, checked the ad and it looks good. I was wondering,since it says they are good for rats and skunks, about how many you do figure we would need for D.C.?