1998 Chevy Tracker, 4-door, 5 speed

I drive 90% highway, 5% gravel, 5% offroad

What options do I have to put a locking or LSD differential in the front or rear? I need to consider all options, which as it turns out are not that many.....

   ARB is my personal choice,from auto lockers to air lockers,they is good.they offer products for your truck in 3 models and start at about $250.

If most of your driving is on-road i would stick with a positraction unit. I have a power brute in the rear of my 95 tracker and it works great.
It seems to be locked when going straight and the tries chirp going around corners a little when the clutches disengage. I have it over a year and no problems.
You have to know how to properly set it up or have a shop do it for you.