a low pinion has the pinion mounted below the center-line of the axle, on the lower 1/2 of the ring gear, putting the pinion closer to the ground and a lil farther away from the t/case output... a high-pinion is the same housing "flipped over" so the pinuion is mounted above the axle center-line on the upper 1/2 of the ring gear, increasing ground clearance and putting the pinion on a close "plane" to the t/case output...
mike