With my kind of wheeling.... (flexy trails, rocks, logs etc.) Id go selectable front if you have a choice. Just remember, you do have stock components all around up front. If you already have a 29spline 8.25 then lock it first. A front locker will put more stress up front. If you havent seen a xj steering box rip through the sheet metal yet, look it up. It sucks. If you can afford a selectable locker, then See how much youre going to have left over after install. If its enough to cover bracing, extra shafts (inner and outers). Chromos perferably. Then I'd do the front, if not, just go rear.
BOTH will help you out in many different situations.
Also, are you sure youre ready for a locker? Maybe upgrade you steering or something else first?