I find with mine that if the shock is fully decompressed and with the wheel up, shock rod doesn't spin. If its on wheels - it does and I have to use a large adjustable spanner to hold it in place to tighten up the top nut.
1997, 4.0 LWB X300 - Unlucky car but still a daily driver.
1988, 3.6S XJS - Never ending restoration project.
1994, 3.2S X300 - Accident damaged, October 2015. Gone!
1994, 3.2S XJ40 - Long gone.