I had this problem too with the rear tires ( squatting ). I had replace springs with HD and put in new shocks, ..but the situation remains the same after a week or two. I brought the car to a Frame and Alignment shop, ...it took them a long time to figure out that my sub-frame is slightly bent. They used lots of force to adjust the toe-in situation and now the rear tires looks straight as new. No more changing tires every two years..
Hope this help ....