It sounds like your problem is downstream of the GFCI. If you have done any work recently you should make sure you didn't put a screw thru a wire somewhere. Also, if the manufacturer managed to pinch a wire in a wall or some other location it could have finally rubbed thru the insulation with the vibration caused from your last trip.
Only way I know to track it down is to remove the last outlet in the string then start working your way back toward the GFCI until you can localize the problem to a wire between two of the effected outlets. Once you do that then you have to figure it out from there. Tracing wires can be a difficult process.
You can check resistance but don't expect it to be zero if you are using a good digital meter. Even copper wire has a some resistance.