No offense to anyone and it's not that you aren't getting some good help here but I would go to the pros at Dexter, Redneck, a spring/axle shop or anybody else who does this work regularly. I've had problems and Dexter was terrific at helping me diagnose and gave me a great cure.
Have you checked all the bushings and bolt holes for wear? It doesn't take much misalignment to chew up tires. It sounds like the tow could be off. If it is... what the reason. Maybe you hit a pothole or curb. I'd want to figure out the cause of the problem and then decide if a heavier axle is needed. Craig