240D driveline locknut

[soothappens]

I checked my driveline while I was under my car fixing the shifter and found the spline portion very loose. Up and down side to side type of loose .

There is a big nut that seems to lock down on the shaft . I tried to tighten and loosen it but it wont budge. Is there a bushing that goes under this nut or is it supposed to squeeze the outer shaft tighter similar to an adjustable choke on a shotgun ? I used a 24" cresent wrench seems like somethings wrong here.

Any Ideas I searched but only came across the different sizes needed for swaps and center bearing u joint repair threads.

Thanks in advance .