Actually....31x10.5x15 should fit a 4x4 with no rubbing as I have a set of these on my 2wd and they work fine. I only get a small rub on my driver's side due to my sagging torsion bar, and that's only when I'm at full lock turning left into a sloped driveway(if I slow down it doesn't happen). Once I either crank(.5" will do it) or replace my bars I don't envision any rubbing at all.
Also, the little bit of rub that I get is not the mudflap, it occurs higher in the wheel well on the plastic ridge above the mudflap.
hth

joel

*1996 T100
2wd,long bed w/5speed
Borla Cat-back
*2002 Z28 Convertible
*2002 Lightning
*2005 Trailblazer