You can find a lot of info on XJ's at
www.naxja.org
But beware! Use the Search as much as possible or get ready to be told to do so by the members there. While it is a decent forum loaded w/ good stuff, it isn't as friendly as here.
I would avoid the 2001 as I hear more problems w/ them than the earlier years. It was the last year so I suspect that they were just throwing them together to get them out the door.
Avoid the D35, so keep your eye out for a 8.25 like the KJ's have and try to get the earlier model high pinion D30 front. They changed around 1999 to the low pinion D30.
XJ's make Great wheelers and while I don't think they tow as well as a KJ at the higher weights, they can work.