2004 Armada tire woes

Hope someone can help. Had to replace all 4 of the stock Contis on my '04 Armada ~23,000 miles. I had had a wheel alignment at ~15,000 miles. Now, with ~30,000 miles I am told that the 2 rear tires will need to be replaced in ~4,000 miles (uneven wear on the middle treads). I always make sure the tires are properly and evenly inflated. Could this be due to bad tires or should I have had another alignment done when I replaced the 4 tires? Is this something common to the Armadas?

Any recommendations on other tires that will have a greater life than the Contis or do the Armadas just grind out tires??

Thanks.