I recommend skill and drill work.
Shouler flexion work
You need to retool your pull up to address the problem.
Carl Paoli has some excellent progressions on developing grip strength(GWOD), and hand position
Your Thumbs, The Hook Grip, and Better Pull Up Shoulder Stability Part 1
Your Thumbs, The Hook Grip, and Better Pull Up Shoulder Stability Part 2
Episode 332: You Can Minimize Neck Movements Under Load, Even the Butterfly Kip