Might suggest BB rows, with an extra wide grip and keeping the elbows wide out throughout the pull.. Pull the bar inline with the neck/upper chest, rather than the gut or lower chest, with a middle grip, that most guys use. Any extra wide pulling movement will affect the posterior delts.
One arm DB's also work extremely well with this wide, elbows out row. Some like a 2 second pause at the top position. Your choice. Chins behind the neck or lat pulls (extra wide grip on both) can be rewarding also.
Even hi-pulls affect the rear delts. Rear delts, not being a massive muscle by nature, seems to give most BB'ers a problem. They do act as a supporting unit for most pulls and even overhead presses. Does respond best to heavier work, I have noticed.
Good Luck.