I wanted to install a h/n/t on my 98 before it went away. I did a lot of research on the subject to find out the best answers before I even started.
1. because the flange is centered on the shaft by the splines, not the bolt, it will not vibrate. having the bolt a little off center will not hurt anything
2. check the bolt regularly, because it seems no matter how much loc-tite you use, it will loosen off
3. I see by the fact that that is a 242 t-case, your ONLY option is a hack'n'tap. Others will argue that you can get a kit from Tom Woods complete with drive shaft, but essentially it is a hack'n'tap kit that someone has drilled the hole for you in the shaft. why do you think he wants your old shaft back??.
4. For a cheaper solution to a custom driveshaft, you can buy the flange style adapter from NAPA and use a front cv style shaft from another xj.
I am not here to start an argument, so those that do not believe me can go to
www.NAXJA.org and type sye or hack'n'tap into the search bar and see what you will get. The only variation I have seen is the homebrew one on gojeep.com