I should chime in on this one even though I am not running 38s, however I am running 35s. I should say that I have no problems running them on the HWY nor Offroad thanks to the 3.15 and 4.90 gears and my 3.5L 215HP <img src="/forums/images/graemlins/kewl.gif" alt="" />
My tires are getting worn out and I have been considering moving up to 36s or 37s to see how the truck will handle. I have more then enough power off/On road with 35s so now I wan to see how far I can push the trucks limits... we'll see! I been looking around for a used set of 37s just to try them out and if they don'y work out I can always sell them and move back to 35s.
I don't have clearance issues for 35 and don't think will be a big deal going 36s or 37s.
Here is a pic of my truck with 35s with a 3" BL + 2" rear springs for lift (I can probably get another 2" on the front with the torsion bars if necessary)