Felipe DC

So my 79' 928 5spd, which I'm new to, clunks pretty badly after having shifted into any gear, once letting off the clutch. It also does it when accelerating or decelerating. I notice lots of rotating play at the drive shaft going into the trans, so at the input shaft..? Right where the 'rear' square clamp joins the two, but the play is mainly in the section that would be the input shaft. The drive shaft coming from the front of the car (torque tube?), does not turn when I turn the square clamp, only the rear section turns freely about 1/4 to 1/2 in. and to me seems to the culprit of the clunk.
Do I need ring and pinion gears?
...or is there another component (s) that wear out in that area. Input shaft or driveshaft splines?

Thanks so much in advance for any help and advice guys!
And glad to be a new member of this awesome group 👍🏁