I recommend going on the Tire Rack website. Type in the size tire you want, and start checking out the different specs on the tires. For example, the BFG AT tire showed it weighed 47lbs. I would have to assume they are all going to be in the 42-52lb range.
The higher the letter, load range C, D or E, the stiffer and heavier the tire will be. For our trucks i wouldnt want anything higher than a load range D. Some AT tires come in a load range S, which is even lighter and provide the best ride but may not hold up as well to hard offroading.