Not a fan of the AT81's myself. The best front tires I have used in the desert have been the Maxxis SI and, IMO
THE best, the Goldentyre Fatty. I normally get about a year out of each so well worth their price.
Good rear tires have been the Sedona MX907HP and Shinko 525. I found the perfect rear tire (GT523KX), IMO, but Goldentyre discontinued it.
So, I've been on the hunt again. I think the Kenda Ibex is next up to try.