a simple stick to keep wheels a certain distance apart

[slight]

You should go with calculating distance. I cant see how your code would work as it is..

dL=Math.sqrt(dx1*dx1);

is just a slower way of saying:

dL = Math.abs(dx1);

(and I think only needed if your vehicle turns upside down?)

But how can you know if the wheels are too far apart when treating the x and y seperately?

you need a proper distance calculation, ie.

dL=Math.sqrt(dx1*dx1 + dy1*dy1);