I'm putting new ujoints in the front driveshaft of my 93 XJ. I don't think this thing has ever been apart in 166000 miles. Should I be replacing the centering ball as well? Is there a lube-able version available? How long do those things last anyhow?

Greasables don't last for sh*t unless you lube them every 5k. But as far as the sealed CVs, I've seen them go for a long time (not sure about 166k though).

If it dosen't have any play in the joint...run it!