It is more than likely that your short is to ground and not across the legs. How difficult would it be to isolate the ballasts/fixtures on the poles from the feed? If they are fed from underground and have an access hole at the base, I'd take everything completely apart and test each individual conductor to ground in an isolated environment.
With any luck, your underground wiring will test OK. Hook all of the supply conductors back together and fire up the circuit with no fixtures connected. If the breaker holds with nothing connected, you are in good shape.
From there, it should be a no-brainer by adding the fixtures/ballasts back to the circuit, one at a time until you find the bad one.
Unfortunately, any discharge type lighting is going to provide all kinds of strange readings to ground, so your readings of zero Ohms are fairly normal. You're going to have to do this the hard way.
It is labor-intensive, but in my mind, it is the only truly effective way to trace something like this. It is really no different than tracing an inside fault except for the weather conditions that you are obviously facing this time of year.
Last edited by EV607797; 12/08/11 12:00 AM. Reason: "Hotline" and I appear to be working double duty and he beat me to it.