I hurt my back a couple of weeks ago at work. I am a night stocker at Wal-Mart, they had me go over and help the guys unloading the truck. I didn't have anything happen to me in the truck other than one box hitting me on the neck.
The next day and two weeks since then, my back has been killing me. My lower back down to my ankles on both legs are throbbing, and my middle and upper back goes numb all of the time and sometimes stays numb for two to three hours.

I was told at work that Wal-Mart won't pay because I didn't tell them when the injury happened. From what I can tell, most back injuries happen over a period of time and often a direct injury doesn't always have immediate symptoms.

Do I have any recourse? I haven't been to the doctor yet, I can't afford to.

You can file for WC, but to be honest, if you didn't file the incident when it happened, and since the accident wasn't even to the same part of the body that is now bothering you, I would expect an uphill battle to get it covered. While the effect of an injury may be somewhat delayed, if you didn't mention the incident that you believed caused it until weeks later, it does amke it more difficult. If others witnessed the incident or at least heard you speak of it, it makes it easier. That won't help you with the causal connection issue though.

You should file as soon as possible as there are statutes of limitations on making claims. Most often this is 30 days from the date of the incident.