Great info here, folks. Looks like the rear mount wins.
FishPOET, you have the exact rack I bought. I dunno what the company is, though, cos I bought it used from someone who also bought it used, and there's no name left on it. Did you buy it new? Do you know what kind it is?
irish1371, OR-Fab does not make a swing bracket for the XJ rear. I'm thinking I see a way to add one, though.
GregB, I have emailed with Mark at Detours, but in the end I will probably use a local fab shop like MarsFab to do the work. The receiver mount does not look like a good option for me - I actually don't have a receiver hitch on the back, and even if I did (which I do plan to) I would want to keep it "free" to use for recovery and/or actual towing.
I have to admit I was not fond of the tire on the roof plan, as a 5"2" female it's not exactly easy to manipulate ANY tire much less one up on the roof. However, very temporarily until I can afford to get a proper swingout fabricated, I may throw it up there for space reasons. It's not a daily driver, but it does tend to get called upon for the tough hauling jobs around the house, so space is important. I started this build with 32s, considered 33s briefly, but went back to 31s when I realized they would be easier to find replacements for out in the boonies. I'm not building this to climb vertical walls, just to take fun long trips through rougher country.
Black_ZJ, how did you extend the stock tire bracket? Love to see a pic if you have one.