Cross-over steering is when the draglink from the pitman arm to the passenger side knuckle goes above the leaf spring (mounting to a steering arm that is mounted on the top of the knuckle - the knuckle needs to be a "flat top knuckle") and the second drag link goes from the passenger side knuckle to the driver's side knuckle below the leaf spring mounting to the kncukle in the stock location.
High steer means that all of the steering linkage (drag link) is above the leaf springs and mounts to a steering arm that is mounted on the top of each knuckle. Each knuckle needs to be a flat top knuckle and they need to be machined, drilled and tapped for the steering arms. The passenger side steering arm will have two holes in it for the steering drag link and the driver's side steering arm will only have one. Take a look in the off-road section as there are some decent thread about highsteer systems.
This s a pretty good article about flat top knuckles and high steer - http://77cj.littlekeylime.com/flatop_knucles.html
This is what a cross-over steering system looks like (it's on a D60 front axle but the same rules apply):
This is what a high steer system looks like:
This here is what a flat top knuckle looks like (passenger side and drivers side w/steering arms):
This is what a flat top knuckle (left) and a non-flat top knuckle (right)look like side by side: