1998expy, like was mentioned, the biggest tire that you can fit on your truck is a 285/70R17 and be garanteed no rubbing. Now, with that being said, some have managed to step up to 295/70R17's and been just fine (probably because they cranked the torsion bars slightly).
With this being said, I ran 315/70R16's (aka, 34.6x12.5's) for a month on the stock suspension. It drives down the highway just fine, but if you start to twist the truck up, YOU WILL!!!!!!! have rubbing. Granted, with a few mods, you can make them work. But the mods would be say a 3" body lift or doing the wheel well mod (ie, taking a 3 pound hammer to the back of the front wheel wells, yes I have done this to my truck, talk to me if you are contemplating this).
How adventurous are you feeling? Oh by the way, I am now running 37x12.5's with only a 4" suspension lift. This is as big as one can go. TRUST ME!!!!!!!! Going bigger will require far more modification.