Glad to hear the lug nuts were tight and you got it to a facility safely. A separated tire could be the problem, but usually the symptoms are a reverse of what your described, wobbly at low speeds and okay to hiway speeds. A defective tire is still a possibility, however. Be sure to get Pep Boys to commit to diagnose the problem and solve it in one step, not just continually throwing parts at it until they hit the problem. They have been known to do that a lot here in Sacto.

Good luck,
Richard B.