Seruriti, from the pics it looks like you might have an issue with the differential making contact with the carrier bearing going to the 2nd. Maybe you don't get the truck in that rough of terrain, or it doesn't squat that much. Shoot, it would probably take 6k pounds to make the factory rear suspension squat that much. Just an observation.
Also, I second the u-joint angle from t-case 1 to t-case 2. A double-cardan joint would be a better choice at that location. The angle looks extreme in the picture.
I bet the rear wants to spin because of the suspension design. If the arms that are locating the axle were ahead of the axle then as the axle drove forward it would also be pushed downward. Since they are behind the axle, as you power the axle will want to lift.
Think of it like this... Sit in an office chair and push yourself backward with your feet, then try to pull yourself forward; your feet slide on the ground more. That's my theory anyway, or you don't have enough air in the bags :B
