If I change the rear differential gear ratio, do I also need to change the front transfer case or di...

I don't see why you shouldn't have to.  The fronts and rears kind of like to go the same speed...

That was my thought, but being a 4wd newbie, thought I'd ask for some experienced advice.

YES you have to change them, or else you will screw up your t-case bad.  my friend got a whole new axle off some one instead of re gearing it, it was cheaper. he has to get gears to match his ford 8.8 in rear

Terrence would be correct on that one. On most 4wd vehicles they advise replace 4 tires at a time for the same reason...(Different tread wear will result in a minute change in final drive ratio)

Both diffs have to match, I know of a chart, i think its in the Jeep owners Bible, tells best gear ratio according to tire size.  you are best off leaving it stock for "STREET Performance"  ( =P )

OMGawd! this is old!  aug 2007 =P

if you do not change them both and make them the same ratio the jeep prob wont even move or the axle or t-cace will destroy itself