Hey... one of the new Calgary teams! Welcome aboard, and welcome to Chief Delphi. I'm hoping to get out to Calgary this year, and look forward to seeing your machine.
We had this problem with our first robot, a four wheel drive machine... it worked great on our cement and linoleum floors at school, but when we finally got to Toronto and put it on carpet... we couldn't turn worth a darn.
The problem is inherent to skid steering... the front and rear of the robot needs to skid sideways, in order for the robot to turn. Since we design the robots to have high traction wheels (which were great for pushing) they were awful for turning. We had to reduce the coefficient of friction of one set of wheels so that they could slide sideways.
With a six or eight wheel drive (which we eventually switched to) the trick is to lower the centre wheels... this reduces the "normal force" (hey, physics in action) on the front and rear wheels, making it easier for them to skid sideways.
If you are locked into a four wheel drive design, you might want to consider some omni wheels...
http://www.andymark.com/Omni-s/51.htm they also work great with six and eight wheel drive machines.
Good luck! Hope you get the machine running well soon.
Jason