If the 2nd gen Vitara with a 2 liter engine came with the air actuated front diff and the 5.12 gears you desire then yes, they should swap right out. Yes, I mean, unbolt one and bolt the other in its place (Suzuki part for Suzuki part). The bolt pattern to the axle housing and third should match as well as the CV axles. That would be the cleanest way to gain the higher mechanical advantage you are looking for. I'm sure the 2 door '99 Tracker with the 1.6 Liter came with the 5.12 gear sets.... I wasn't aware the 2 door Vitara with a 2 liter came with 5.12 gears. Makes me wonder now what transmission the Vitara are running simply because I would think a 2 Liter engine should pull stock 27 inch wheels without the higher mechanical advantage of 5.12s. Hmmmm? I was thinking the 2nd gen 2 door Vitaras came with factory 4.88 gears.
Anyway, IanL frequently validates part info here on the forum (and helps keep us both honest and straight). He may verify the info (axle ratios, year, part interchangeability, etc).