For AT tires, it doesn't get much better than the Goodyear Duratrac. If you want to get a little bit of your performance back, you could change to a 33" tire. However, the 35" Duratrac is going to be much less weight than the Nittos. So even switching to them will get you a bit of performance back.
However, keep in mind that any tires for those stock 18" wheels are gonna hit the wallet HARD! I might would recommend selling the wheels as well and going with something in the 15"-17" range.