Truck pulling to right.

Two weeks ago, i had lower ball joints installed and an alignment on my truck,after that, it pulled to the left a little, took it back and told them, they hooked the alignment machine up and said that it was dead on. So the tech said it could be a tire issue, they switched the front tires, and now it pulls to the right, so thinking that it had a bad tire i replaced the right front tire,still pulls to the right, just came back from sears and the alignment was off, had them align it and it still pulls to the right, what could possibly be wrong, did i replace the wrong tire, or what. If anybody has any ideas please let me know. Thanks.