What differences are there from a 99' Disco I to the previous years? I've noticed a few pictures in the -10 in the glovebox and some I have seen online where my Disco is different than the years before it.

 

Trim levels, 99 got more for the money so people would buy the Disco I model as the DiscoII was on the way.
Emissions 97 to 99 same,94 to 96 different emissions with different fuel pump.
96 started withfour O2 sensors 95 and earlier hastwo O2 sensors.
That covers most of the things that come to mind.
Head lights same from 96 to 99.
The body was the same.