Hopefully I read everything correctly. You stated that you're running a 6WD bot, which is great!... but you need to consider a few things that have been mentioned already. First off, when everyone says to lower the middle wheel, you really need to do this somehow. We've gotten away with it in the past with pneumatic wheels that we could air up to raise and lower the center of the robot. Very good solution might I say.
That is a quick sketch of a lowered centered wheel. I believe we've used about a 1/8" drop which has worked great for years.
If you can remove the front wheels, and the robot maintains stability, then you have a balance problem. The entire weight of the robot should remain mostly on the center wheels, with very little weight on the outer wheels. You said that you completely removed the front wheels, which leads me to think you have way too much weight in the back. If you haven't already, you should move that roughly 12lb ballast called a battery to the far front, and if that's not enough, the other ballast (cRIO) may work out, as well as most of the other electrical components.... or if you have spare weight, just add some weight to the front.
Like Chuck said, make sure you're not in high gear... although it should be able to turn in high gear, but it would be a very easy programming fix that could shift it into low gear when the joysticks are the inverse of each other. (one is positive, one is negative).
Good luck! I hope you fix it! I know it stinks to be this late in the season when the design is coming together, just to find out that you have a major problem like this. Every team has probably been there at some point, I know ours has. Just think it through, get creative. I do recommend a pneumatic wheel if your design permits. Remember when you're turning, the outer wheels do nothing... except bind the robot.