Regarding the 6x6 questions.- A "normal" 6x6 does not have the axles spaced evenly. To do this would require a very complex steering system, OR skid steer.
To deliver the power to all three axles- Much like a 4x4, a 6x6 uses a transfer case connected to the transmission. The transfer case has outputs both front and rear, for the driveshafts. The forward driveshaft simply goes forward from the T-case to the front axle. The rearward shaft goes back to the first rear axle. Then- here's the trick-, the differential in that axle actually has double pinion yokes, one in and one out. The output pinion yoke points out the back towards the second axle. A driveshaft connects the two.
As far as skid steer goes, there are several ways to achieve it. The simplest is not the best, but is by far easiest and cheapest. This assumes your vehicle has standard, open differentials with no locking mechanism or anti-slip. The braking system must be re-plumbed with two master cylinders, one each for left and right brakes.
To steer left, apply left brake. To steer right, apply right brake. Simple. Again, not the best, but functional.
@Redsox- How to determine an "open" differential: Jack up the vehicle so that BOTH tires off the ground. Turn one by hand. If the other turns the opposite way, it is an open diff. Make sure the driveshaft is not turning (IE vehicle in park, or in gear if standard).
