Poprivit
There's more to think about here. The rear radius rods attach to the torque tube. When you convert to open drive, there is no support so another solution has to be found. You need to attach the ends of the radius rods to something like a crossmember (you add), or some have actually made the rear radius rods into a wishbone, with an attending mount. Further, as Jim above has mentioned, it is generally wise to add an additional torque arm to the rear end. Now, in answer to your original question, when the suspension moves up and down, the sliding yoke (front U joint) moves in and out of the transmission.
Jim