I ounces had this problem with an truck a wail back and found to be a combo of things that gear box at the bottom of the column I found the bracket that held it on had broke free from frame. u will need to jack up rear wheels and look down with a flash light with wheel cut al the way one way. Be careful block the truck up The gear box also has a adjustment screw on it to adjust free ply and gear smash. After that u will need to inspect the drag link there is a tie rod looking end and is actually a valve and ports oil and that is the HYD steering then check all 4 tie rod ends and spider and drag link ounces all that u shod had power steering again once all ply is repaird

I should have mentioned that about 40% of the steering wheel play is the PowerSteering ram actuation, so when the motor is running the play is much less but still not real good.
I have degreased around the spline shaft & there is a C clip at the top that holds (stops it falling off is about all) the shaft on the spline.

Maybe the arm (female spline) could be slit & blocks with bolt holes welded on to make it clamp the shaft. This would require specialist welding being cast/forged.