BoulderXJ

I was checking caster in my 2001 XJ using this method:
http://go.jeep-xj.info/HowtoAlignment.htm

I started to get pretty confused because my diff measured at 90 degrees. On a Dana LP 30 that would be way off. I looked a little harder and discovered that I have an HP 30 in my 2001. So, first question, did any 2001 XJ's come with the HP 30, or must the PO have swapped it?

I have adjustable upper and lower control arms (JKS w/ 4.5" lift, no drop brackets), and originally set the lengths based on this:
http://www.yuccaman.com/jeep/re_db.html
Using these lengths gives me 90 degrees on the diff, which on the HP 30 is a 9 degree caster angle based on extrapolating the table. I shortened up my lower CA to 16.25 and am now down to a 7.5 degree caster angle. I'm thinking about now lengthening my upper CA's to 15.25 which should give me around 6 degree caster. BTW - I do have a 1" transfer case drop so the driveline angle should be improved slightly.

So, why the discrepancy? Is it because I have an HP 30 in my 2001? Did the axle and motor mount locations change such that the control arm lengths would be different if you use this axle and model year combination? Or is this simply fairly standard build variation and not too surprising that I have to go .25" longer on UCAs and .25" shorter on LCA's?

Last question, what happens if I leave the 7.5 degree caster angle? Is that bad, what is the effect on handling? I've searched for this but caster shows up in so many threads I've had a hard time finding a good explanation of the effects of too much vs too little caster.

BTW - part of why I've been messing around is to get rid of a mild vibration at 55mph.

Thanks for the help!