Why did Rick end up going to jail?

He told the girl that he was going up-state soon, which is a colloquialism for going to prison. Also, he said he'd give her a call in 12 to 18 months.

I'm guessing the most likely crime he'd been found guilty of would be tax-evasion.

The allegations of corruption would probably be too difficult to prove as witness credibility would definitely be an issue with most (if not all) of the complainants being criminals themselves, no cops would ever testify against him, and no jury would rush to convict him of that after saving a woman's life.

The tax-evasion would be impossible to argue against in court seeing how he was driving away in that expensive-looking sports car in the end, so my guess would be that he took a plea-bargain in order to receive a more favorable sentencing and perhaps prison choice as well. Considering how he was a cop, I think the prosecutor might actually give him a good deal.