Which vs. That

Q. Could you please explain to me the proper usage of “which” vs. “that”? I could really use a “hard-and-fast” rule to keep in mind regarding proper usage of these terms. Here is an example of the actual sentence currently in debate:

The Economic Growth and Tax Relief Reconciliation Act of 2001 which became law on June 7, 2001 was the largest change in tax regulations in over two decades.

I felt that in this instance “which” should be replaced with “that,” or that the phrase “which became law on June 7, 2001” should be set off in commas. A coworker disagreed, saying that “which” is correct because there is only one Economic Growth and Tax Relief Reconciliation Act of 2001, but that doesn’t seem right to me. Should we just have rewritten it to say “The Economic Growth and Tax Relief Reconciliation Act of 2001 passed on June 7, 2001 was the largest change in tax regulations in over two decades”?

HELP! Thank you.