there are a few possibilities that you can check
first, if its felt in the seats the usual cause is the back tires. they may have a wear pattern in them from being out of balance the, even though the tires were re-balanced you will get the vibration from the wear into the tires. that condition is called cupping. to see if that is your problem, rotate your back tires to the front and see if the vibration either goes away or changes.
second thought is that you may have a torn or broken motor mount. that is extremely common on the accords and that will cause the vibration on acceleration and deceleration. it is usually the front motor mount that breaks.
my guess is that your problem is with an engine or transmission mount.
hope that helps