OP needs to describe the issue more. The FX is absolutely not sliding, it's a tank. I cannot get the tires to break free no matter what. If you push hard enough, the front end just starts pushing in a turn. Even if you try to do something dumb like take a 90 degree turn too fast while staying on the throttle, the FX won't slide. It will lose traction for a tiny second, then traction control kicks in and you start understeering again.
If the OP is referring to the way the rear end of the FX is very unstable when going over bumps, that's just the way it is. Underdamped rear shocks + heavy car = weird twisting/bouncing motion. If your FX has a few miles on it, the feeling will be worse since the shocks are more worn. But I had this problem when I bought mine at 12K. Short of coil overs, no real fix. Just get used to it, you won't crash.
And FWIW, I didn't notice this on the test drive either. You really don't go crashing through bumpy corners on a test drive. I found out after I purchased on a specific intersection by my loft. The issue isn't a big deal since it doesn't happen all the time.