I echo Louie's comment. Trading in Coupon Connection is probably your best option. You have to have a minimum number of posts on FT to use it. Probably there is someone out there with United miles who wishes he had Delta miles.
One issue is that you will probably have to trade in the form of redeemed awards. You can't move your DL miles to his account and his UA miles to your account, without fees almost as much as the miles are worth. You basically have to have an award you want to redeem, and have the other party be in the same boat. If you have a Chase rewards account, they can transfer their Chase points to you, and you can hold them until you're ready to redeem, and in effect the other party has been able to transfer their Chase points to DL.