The Cherokees do have them mounted underneath the AC compressor for whatever reason, and for daily driving, its fine. One thing I can tell you is stay away from the Alberta mud. Its wicked silty here and will eat parts like cancer. If you are responsible on the trails, keeping out of deep water when possible, as well as deep mud, you should likely never have any problems with your alternator, even being mounted as low as it is. If you want to carry a spare, for $20.00 at pick-n-pull you have that peice of mind, but honestly, I would carry a first aid kit, fire extinguisher (not your dinky kitchen model), and perhaps a spare axle for your XJ before putting an extra alternator in the back. Thats just my 2 cents
If you keep the underside of your rig clean it shouldn't bear any problems on the trails. Mine has been running fine since 2001 and its given me no grief. Mclean is absolutely notorious for having silty, sandy mud, and if thats where you would like to play this summer as your pm suggested, I would be sure to wash your jeep down after wheeling, as that couple dollars at the car wash, will save you hundreds later.
And of course, welcome to the CJA forum