2 pc drive shaft

[78-5.3]

i got my drive shaft back today and installed it ! the rear shaft has a screw cap that locks the front and rear together !! if tighten that cap does it stop the shaft from sliding on the splines ?? or is it still able to move !!!!!!!!!!!!!! i had to loosen it to attach the U joint to the rear end then it slide back what i need then i tighten it back up !! is this correct!!!!