Yeah guys, you are all correct. There could be loads of things that would cause the problem. Differant size tyres, axle not centred properly, tweak not set correctly, twisted chassis or motor pod. You could also have a problem with the way the front end has been put together as well, causing a problem at the rear.
The only real way to be able to sort it out is to completly strip the chassis down and rebuild it back up.
I always take alot of time when it comes to the building of my chassis's so that when I get to a meeting, all I have to is worry about my driving standard as appose to finding out what's wrong with my car.
If you are not sure about what you are doing, I would suggest asking one of your club members who would be more than happy to help, hopefully!