[Wheel Colliders] Having trouble trying to control slip, weird behavior.

Hey there, I'm having some trouble with controlling the slip of Wheel Colliders.
I've made a forklift with wheel colliders that's steerable by dragging the screen while driving.

All seems well when you drive it normally and don't steer to fast, but when you really jank on the steering wheel, putting the wheel collider at a 70 degree angle, the wheel starts "slipping"...
For some reason the wheel seems to be going backwards, while I exert forward torque on the wheel.
I can't tell which way the wheel is rotating, because in the editor the wheel seems to be spazzing out...

I can't figure out what is causing this, though I have a feeling the sudden rotation of the wheel has something to do with it... I've tried changing the friction values of the wheels, trying to make the wheels never slip (I don't want the forklift to slip in the simulation I'm making), but no matter what combination of values I use I keep running into the same problem.
And, yes, I have looked at the documentation of wheel colliders and wheel friction curves, but am having a little trouble understanding them...

Does anyone have any pointers of where I should look to stop the wheels from spinning out?

Here's a capture of the problem:
http://imgur.com/a/ew1p3

 

Maybe you can fix it by using custom wheel colliders.