A.,
Most importantly (IMHO) stop saying that you are doing something wrong. Seriously, from what you've written it sounds like you have done everything that you can. I know, something obviously isn't working and I'm sure you're frustrated - but, please, don't blame yourself for it.
It seems that your son's aggression is directed mainly towards you. That's the part that confuses me. If he was an over-aggressive child all around, he wouldn't be able to behave so well at daycare. I have a 2 year old son in daycare 5 days a week. My hubby and I work full time. Like any other toddler, Jacob has his fits but they aren't directed any more at me or at my husband - we both get the brunt of it equally. However, one of my closest friends is a SAHM and their son (also 2) challenges her WAY more than her husband.
I think part of the issue is that you are the primary care-giver for your child being a SAHM. You are the one that enforces the rules most of the time so you are the one that your kids will choose to challenge most of the time. They are around you more and they figure out how to try and manipulate you. It's not a bad thing, it's actually rather intelligent (I think) that they learn at such a young age what they can try to get away with around different parents.
Even though (like I said) my DH and I both work, Jacob interacts differently with me as opposed to his Dad. My husband can get him to laugh a lot harder and more often than I can. BUT, he seeks me out when he's hurt or tired. I think your son (for whatever reason - and not because you allow it by any means) feels that he can push you much harder.
At this point, I would be as frustrated as you are. The physical nature of his aggression towards you would upset me if I were in your shoes. You mentioned spanking in your post, but I got the impression that it was somewhat of a last resort that you turned to. Your other methods of discipline didn't seem to reflect any sort of violence or aggression - so where he's getting the idea to lash out physically when angry is a little baffling.
I think (as I'm sure you probably know), it may be time for a talk with his doctor to figure out a solution to the problem.
I'm sorry that I don't have any actual suggestions for you, but in all honesty it sounds like you have tried anything that I would suggest anyway.
I really hope that you can get this figured out as I can imagine how difficult it is right now.
Good luck and please keep me (us) posted.
T.