2005 is not long ago at all. Absolutely you should approach them. Remind them who you are, what class you took, why you picked them out to ask for a LOR, and maybe what they had said about you at the time. You may be surprised how well they remember, and they probably have notes archived from the class anyway. Even if it were twenty years ago, it wouldn't be out of line -- three years is nothing.

I'd be in a tougher position -- I've only taken one online course, and although I got a B, I was half a slacker, and I'm pretty sure the professor knows it. I'd have to take an indirect route and find someone on a college faculty through social contacts.

-Gary-