How can I get my promotion back dated if my unit say I need my proceeds that I did at my last unit?

The only one with any incentive to promote you either in the future or the past is going to be you. I don't know what the process is, but sometimes that process is doing someone else's job.

Things to consider:
1) Can you get a promotion through right here right now as you are with your current unit?
2) What is actually the back pay benefit of being promoted retroactively? I know as an officer, that retroactive pay will only go as far back to the point I was in a billet of higher grade. So I was able to get retroactive pay from the DOR since I was already in the higher rank billet. If I was in a lower rank billet prior to promotion and had to move into a new billet to take the new rank, the pay wouldn't follow me until I was slotted in that position.
3) Considering 1 and 2 what is the time table of jumping through hoops for a retroactive promotion relative to the actual pay bump you'd be retroactively entitled to?

My recommendation is to follow the path of least resistance unless you are looking at thousands of dollars in retroactive pay.

What do you mean by "My Board Proceeds?"