yes you can run 18's or 20's the secret is to stay the same tire height
http://www.discounttire.com/dtcs/infoTireMath.dos
heres a link to play with so you can match up what you want.....remember the stock tire on your truck is about 29 1/2" tall which is a 235/75/15...I can run mine as low as it goes with te stock rallies so look for the same back spacing on the rims...I think 4 3/4" is the amount your looking for..then you can put in a 2" lowering block in back and either cut 1 loop off the front springs or go with 2" drop spindles...I think that will give you the look you want....you could also look at what you have and measure from the fender lip down to your tire to get an Idea of how much drop you want.....just stay with the same tire height and it will keep your speedo correct