there will be a sticker on your rear axle tube tellng you what gears you have. see link: http://i647.photobucket.com/albums/u...Axlelabels.jpg
you can also find it on your window sticker too if you boght new.
gears would be kinda expensive b/c you are 4wd but would be the most bang
for your buck depending on what ratio you currently have.
fred also has some good more budget friendly ideas: throttle body, remove your belt driven fan (you have to install the 180 t-stat with this and use your programmer to change your efan on/off settings)