30x9.5-15, minor rubbing on stock height
31x10.5-15, lifted 2"-5" with some trimming
maybe a 32x11.5-15 or 33x12.5-15 if you lift it with a suspension and body lift then hack it.
The gearing is pretty much going to limit you to 31's, a 32 or 33 will be decent but you aren't going to like it for technical stuff or off the line horsepower. If you have an automatic you should avoid overdrive with the 32's and 33's (if you can make them fit)
With that in mind, my ZR2 Blazer is 3" higher than your regular s-series truck and running 31's with 3.73 gears and I can easily rub the front and rear tires with both sway bars connected. The gearing is good with these tires and if I went any bigger I would want at least a 4.10 gear. /forums/images/graemlins/eek.gif