Sounds like Cervical Kyphosis.. Overdeveloped internal rotators, tight shoulder flexors, with weak or stretched external rotators/back muscles.
Rodja hit on it pretty good, The biggest thing that I didn't see mentioned in that article is to be sure your fully retracting your scapula when performing back exercises, and to even perform scapular retraction exercises. Try narrowing your grip on your rows/lat pulldowns, and really pinching the shoulder blades together with every rep.
It's all too common for a person to let their biceps and rear delts overpower their back movements. You also incorporate the internal rotators during movements like lat-pulldown, which is counter-productive... So if the thats you main back movement, then you need to switch it up
when you maximize muscular development, the internal rotators are stronger, and contain more overall muscle tissue than the external rotators... so there will generally always be some internal/external imbalance in bodybuilders. However, there is no reason for the shoulders to slump or roll forward
My Rowing movements are roughly 95-105% of my pressing movements, depending on what exercise were talking, and that keeps my shoulders drawn back, and posture remotely decent despite some internal rotation of the humerus