crawling is more than being able to climb over a carefully arranged sequence of objects.... a solid axle means that you dont have arms in the suspension, the wheels are held rigidly to the gearbox and its the gearbox that moves

http://en.wikipedia.org/wiki/Solid_axle

one advantage of solid axle is that the vehicle would act more like a pair of lines, like one line representing the front wheels and another line representing the rear wheels... in an independant suspension setup like what you have in your 18mt, the whole vehicle would hold a rectangular shape and it would not be flexible enough for true rock crawling