From what I've heard the wiring harness has a different "plug(?)" count, but you can just snip and solder. Also check the lenth of the shaft.
I'm also considering upping to a 23 spline when the ol' motor gets stroked. But the real question is will a mildly stroked motor snap a 21 spline shaft? I don't *think* so...Has anyone seen this happen? I'd rather not deal with it unless it's necessary. I'd like the extra power for mountain passes, but I won't be competing in side shows with the rice burners.
Besides, all the torque reduction is done after the tranny output shaft, as you get further down the drive train, as in the t-case, (or in xjnations case, the K-V AND the t-case) that's where the torque happens.