Spring pivots are on your original axle because you have a torque tube. With a torque tube there is one u-joint (transmission end) and the pivot points allow the back axle to swivel when going over bumps and such.
If you put in an open drive you will have 2 u-joints and consequently you will have to weld the pivot points otherwise the u-joint will clash with the rear pivot.
Look at your axle and you will see where the spring joins the axle that there is a pin sitting between the u-bolts which hold the axle and spring together.
If you notice your rear wheels are not centered in the well. That is because with the pivot the axle/wheels can move and they can go forward or backward albeit not much, when you go over the bumps.
Hope this helps.
