[MR Zook]

what mechanically changed in the vitara from year 98 to 99 that would prevent me from using the Calmini 2.5 inch lift kit and why, couldn't find the the answer online.

[fordem]

Calmini is a US manufacturer, so the model year designations shown on their website are based on what was offered in the US - you're in Australia, where the vehicles available in those years were different to those in the US.

In a nutshell, there was no Vitara sold in the US prior to 1999, just Sidekicks & Trackers and when they say 98 or earlier, they are referring to the SE series (SE416), and a 99 or later (99~05) is an SQ series (SQ416/SQ420/SQ625).

There were many changes, too many to mention, but all you need to know right now is that Calmini offers lift kits for both SE & SQ so once you know what vehicle you have, you can determine which kit fits it. There is also the SV series (95~98 - SV418/SV420/SV620) which I don't think they make a kit for.

[Gwagensteve]

The only relevant differences between SE (1.6) and SV (2.0) vitaras for the purposes of the calmini kit are

Diff drop brackets are different and require modification. You may be able to buy SV drop brackets elsewhere.
Front A arms don't have removable ball joints so you'll need to buy two 1.6 front lower ball joints.

[fordem]

I was under the impression that the SV arms were longer than the SE ones.

[Gwagensteve]

I don't believe this is the case- all the extra width is in chassis rail spacing AFAIK.
I have swapped calmini SE arms onto several SV's without issue other than needing to procure SE lower ball joints.