Here's a review on the Dune Grapplers ... sounds like they're good on dry pavement and sand but not much else.
Lots of other tires too including the BFG AT KO.
http://www.offroaders.com/tech/AT-MT...e_Grappler.htm
These, PJ Dirt Grips, look as good in the sand but better overall, closest size is 37X12.50R20, but they do have them in 35x12.5 and 33x12.5 for both 17" and 18" rim sizes:
http://www.offroaders.com/tech/AT-MT...-dirt-grip.htm