"We have a multi-band scanner at work that picks up portable phone conversations.

... Clearly illegal activities are discussed in these conversations.

Is this a violation of an individual's rights to inform the police?"

This sounds like a tough call, however, the federal law is clear. No eavesdropping on any telephone without a court order. The full text of the law can be found at:
http://www.cpsr.org/

The reason this "sounds like a tough call" is because the law tries to solve the privacy issue from a counter-intuitive approach. ... Imagine a law which requires us to put our hands over our ears so we don't hear: profanity in public places; a person shouting fire in a movie theater; or construction noise after 10 pm in a residential neighborhood.

There is nothing you can do - because of the way the law is written - to assist law enforcement in their duties. Very unfortunate in this case.

Kevin