The salient facts of the case are this:
1. I went through a very ugly separation about 18 months ago. 1 Child was involved.
2. My ex-wife moved, with my daughter, approximately 8 hours away from our home in Jacksonville, FL into Georgia.
3. I was paying child support of approximately $450 per month through payroll deduction.
4. Due to the child support payments and a suicide attempt, I was evicted from my apartment and moved in with my brother.
5. In March of 2009, I received a final divorce decree in the mail at my brother's stating that my divorce had been finalized, my child support raised to $747 per month, and that I would henceforth be allowed to visit my daughter only if I drove the entire way to my ex-wife's city (instead of meeting halfway which had previously been the agreement), and that I could only have visitation if my mother came with me to pick my daughter up at the police station. The decree stated that the court attempted to inform me at the address they had on file, and that I was not represented because they could not find me. Remember, this decree was sent in the mail to my brother's house, where I had been living the whole time.

My question is this: since my interests were not represented at the hearing due to neither the plaintiff nor the court took what I believe to be reasonable measures to locate me (i.e. calling my known relatives), do I have grounds to hire an attorney and attempt to get the terms of the divorce changed (possibly a reduction in support payments, or at least more favorable visitation rights)? Or have I been hosed with no chance of a more equitable arrangement?

You could fight it. Did you tell the courts that you changed addresses? Did you have a lawyer at the time? What state is doing the child support and the divorce?

I did not notify the court of a change of address. In the state of Florida, for certain hearings, if one of the parties is not going to be present, the attorney for the other party must submit an Affadavit of Diligent Search, certifying that they took reasonable measures to locate the absent party. I believe that calling my relatives, whose names, numbers and addresses were known to my ex-wife, would constitute a reasonable measure. Most judges in the state of Florida agree, and will throw out the Affadavit if it does not specify that known locateable relatives were contacted. I don't know if Georgia has a similar law, or how to find out.

We both were residents of Florida, but she moved to Georgia and stayed long enough to establish residency prior to filing for divorce in Georgia. I did not and still don't really have the money to hire a lawyer, but am thinking about hiring one, nonetheless.

Was anything filed when she left? Separation papers etc? What state was the child born in?

She filed for a separate maintenance agreement, and I was personally served with the papers when I went to visit my daughter on that one. Faith (my daughter) was born in Florida, which doesn't recognize a legal separation. I likely was required to inform the court of a change of address, which I failed to do. With a hospitalization due to a suicide attempt, eviction from my home, and a car repossesion (mostly due to a mistake in my income deduction; the payroll company thought that they were supposed to be deducting $2300 per month and deducted exactly half of my pay each paycheck), it slipped my mind. I'm just curious as to whether a diligent search is required for divorce proceedings. In my opinion (which doesn't necessarily reflect the court's opinion), no diligent search was conducted. Seriously, they could find me to send me the order telling me how bad I had been hosed, but they couldn't find me to inform me of the divorce hearing?

Then get a lawyer and contest it.

I'm consulting with a lawyer about my other issue (which you've also been posting on), and I'll bring it up with him.

The issue here is first how long they lived in GA before filing, I believe they had to live there 6 months to gain residence before filing.

Next the attempt to locate varies, but they would have hired a process sever, who may have only posted notice at your last known address. And the process, may well have been valid by GA law. ( I did a lot of process service in GA part time)
Remember if GA has the right to do the case, FL law means nothing, GA law is what rules the case.

In GA, your child support is set by income, there is a forumla, so if she used your valid income, then that may not have any bearing.

The fact you went months without trying to see the child, is going to hurt you in most GA courts, you are going in against a stacked house if you can't show attempts to see the child.

I do have one question, was there already a child custody and support agreement in place, you said you were making payments, if there was, it may actually be the valid order, and a new agreement can not be ordered in GA.