Took forever finding right lower rear control arm. No one had in stock except for Jaguar original for $1000. Found on Amazon in mid September. Keep getting message shipping delayed. Came back in stock on RockAuto and arrived this week.
Ball joint takes a 6mm allen to hold while tightening nut. One side went on fine. Other side rounded out. Used a pry bar to put on pressure as pictured and an air impact gun.

 

I just had the same problem with the lower control arms from RockAuto. Spinning on the spindle. I temporarily installed the brake rotor and put wooden block under it. I then used a small jack on the upper control arm to force the lower ball joint to lock on the taper. After tightening the lower balljoint it looked as if the thread on the nut was slightly off as the exposed threads on the ball jointsa looked a little askew. However, I was able to tighten the baljoint to spec. Hope you put some threadlock on the torque bolts holding the 2 lower arms together.

 

Jag manual didn’t mention thread lock so I didn’t. I’ll check still tight next time on lift for oil change.

For anyone doing this work, I didn’t loosen steering rack per manual. Loosening bellows on rack and turning steering wheel this way and that gave all access needed.

Also, for reassembly: 1. insert lower ball joint; 2. rear arm to frame (don’t tighten); 3. front arm to frame; and 4. bolt front and rear arms together. Much easier than assembling parts first and trying to mount.

 

you shouls have seen some blue goop on the threads of the torque bolts. That means that upon assembly some threadlocker was used at least on my car. I put some thread locker on mine during the installation process. Better safe than sorry. It won't be my job to remove them anytime soon. I love my XK8 and plan to drive the hell out of it. Still a few more minor things to do on it.