Up sh*t creek...

Have you talked to the neighbours?? Maybe they have a similar problem? If so it might not even be "your problem". Wtih all municipalities I have worked for, it is the home owners responsibility to the curbstop for water, and the property line for wastewater. Plug could be down stream from your "responsibility" Good luck.

That was actually my first thought. The city has been doing a LOT of sewer work in my area. Problem is, all of my neighbors know we have a clog, but none of them seem to have one as well. My wife looked it up online, and we can call our city sewer works and have them come out and check the city line to make sure it's not clogged where it meets our lateral. If it is, they will clean it out for free. If not, back to the drawing board.