Hi,
Not sure if would be same for you, but I have a similar problem with shoulders hunching up. Had a lesson with Ben Lawry few weeks ago, and for me the problem was actually the elbow of my top hand was rotated too far up/out. As I brought my top arm across my body, my corresponding shoulder would hunch up. Ben had me rotate my elbow more down towards the deck (about 90 degrees for me), and instantly the mechanics of my shoulder would now stay incredibly flatter as my top hand moved across my body. This also helps me keep my top hand higher as I finish the stroke due to my shoulder not "binding" at the end.
Have no idea if this is of help, but sure has worked great for me!! I'm a huge fan of getting a lesson (with video as Cam mentions)if you have access to great instructors like Ben.
Good luck!!
Chris