My lease in Florida included an early lease termination/liquidated damages addendum. I signed it in haste not realizing that I would owe 2 months rent if I moved out before the lease expired. Unfortunately I had to move early due to job loss. The property management company is expecting this money from me. But they kept my full deposit and had also leased the rental to someone else right after I moved out, so it is not like they are out any money. I don't have the money and they know that. What can they do to me legally? Will my credit be ruined?

You are lucky in many ways.
1. Flordia landlords do not have to even try to rent out and could expect full payment for all the months. This was actually a good lease for a renter only owing two months.

2. Since this amount is a pentalty to allow you out, you are bound by the terms. You should not lose your deposit also, unless that was also written in. ( or the deposit should apply to that amount after any damages are deducted)

They may sue you, I would get the proof that someone else moved in. if you go to court, how do you prove this to the judge ? And then wait for them to sue.
In the waiting, they may turn it over to collections and ruin your credit

What exactly does the addendum say? An addendum is an afterthought to an original document, usually signed separately. So, if you signed this separately, I am surprised that you didn't realize what it said.

As Chuck said, this addendum actually was in your favor. But now works against you because it becomes a penalty of the lease. So the question now is whether they sue you or not. If they do, you argue that they weren't harmed because they didn't suffer any vacancy. Not sure if that will fly in court, but its your only hope.

It was included with the lease, not separate. It was a rushed signing and was not explained to me. It stated early termination fee was 2 months rent and owner waives the right to pursue remainder of the lease.

I don't believe this was in my favor at all. Florida rentals are rented in a matter of days, so in fact, I am the one who loses in this situation and he collects my deposit plus 2 months rent.

I don't know what they think they can get out of someone who is without savings or a job. Things happen in these hard times and exceptions can be made if one has chooses to be compassionate.

... Florida rentals are rented in a matter of days, ...

Not all the time.

If it had not rented again, you would have benefitted from the "addendum".

If you are familiar with the current rental market in Florida, or have been looking for a rental yourself, you would know how quickly they get rented mainly due to the high volume of foreclosures. If one is interested in a rental, to hesitate is to lose it, sometimes within hours. This is just the way it is in FL. I know this from first-hand experience and from several real estate agent friends.

Can this fee be used against me on my credit report?

This may be true in your market, but I doubt is it applies all over FL. But the important issue here is FL landlords are not required to make an effort to re-let. So they could simply hold you to the balance of the lease.

Another factor here is you signed a contract for a length of time. What makes you think you have a right to break that contract without penalty?

Yes, the could put this on your credit report, especially if they sue and get a judgment.

Again, you as a NY resident have no idea what you are talking about regarding the rental market.

I do not think I have a right to break a contract but I also did not foresee becoming jobless. I have a stellar 800+ credit history, my rents have always been paid on time and I am in no way some deadbeat trying to slither out of a contract, as you sound like you are implying.

Once again, no monies have been lost, no damage has been done to the owner.

I don't have to be in FL to understand the markets. While some area do have an extremely low vacancy rate, that; true all over FL.

No one is saying you are a deadbeat, But the fact remains, that you signed a contract and are bound by the terms of the contract. Its not the landlords fault that you lost your job.

They don't have to care. The law is on their side. And your question was about the law.