A chain will only encourage her to rear more. In your ground work, do nothing but focus on sending her. Send her forward here, send her forward there, and so on. Stay away for awhile from the backing up, or any other backward moving elements. Sounds like your guy has gotten stuck backwards and his only answer away from backing up is to rear. So you have to be quick to send her so that she learns she ca softly move forward no matter what she's asked of. If you're practicing, and she does rear, if at all possible, do not pull on her head. Just get around to the side of her and twirl your rope at her until she answers you by going forward. She may react by jumping away from you and once again going backwards, but just keep twirling until she makes an effort to go forward. She may just run sideways, stil, twirl until she goes forward, then release everything. No matter what she does, only release on her going forward. :)
If that is not the issue, then you will fix the act of enthusiasm by immediately correcting the behavior by sending her forward. Send her forward enough that the messege is absolutely clear.
Good luck! Keep us updated. See if that helps.