I wouldn't be as concerned with the load index as with the max weight each tire can handle. Doing a quick search on tirerack.com I looked at the Continental ExtremeWinterContact in the size you want and the Max Load is 2403 lbs per tire, the Xterra is not nearly that heavy. Seems the Continentals are the only studless winter tire tirerack.com has in that size @ $103.00 each. Shipping is usually about $15/tire and they generally ship same day.

GVWR does not equal GCWR (Gross Combined Weight Ratio).

-Michael