I assume you've done an alignment and the specs are perfect? Beyond that, the first thing most people blame is the tires, so don't give up on that even if they look fine, etc. Swap front to back to see what happens, for example.

The symptoms you describe could also be caused by a rear camber problem. I've heard others describe slightly bent delta links (trailing arms) as the source of the problem. If so, these are expensive to replace. Good luck.