Sub-frame has to be dropped to get gas tank out in S14s.
The axle question will depend on which axles your differential half-shafts the KAAZ is in uses; 6 bolt is usually the norm so if your S14 now is running a KAAZ just swap differentials.
However since you are cleaning the gas tank out (which I would replace anyway since they're notorious for breaking) might as well get sub-frame spacers and swap the entire rearend from your 97 car in, since the differential/axles and everything would already be on it and would prob take you less time to put in, but the weight of the sub-frame with everything on it is pretty heavy ESPECIALLY for one person.

Good luck.