Rancher,
Sam has the easiest way, but if your want to DIY...
It is most very likely leaking at a joint or other connection. No need to dig up the whole line. Hopefully, the PVC line is in 20' joints, and you can start at one end until you find it. You shouldn't have to dig all the way down to the 3' depth at each joint. You should hit wet ground before that. In lieu of hand digging, if you really want to find it quickly yourself, you can get a helper and rent one of those gasoline-powered posthole augers to drill down next to the joint areas. Either one will be work, but the auger will be faster. It won't get any less with time. (Another reason to move to NC...LOL).
Good Luck!
Mike