Can you further damage a surgical wound on your shoulder by pushing a stool out too aggressively?

Physical therapy: Yes you can, if you using your shoulder. Discuss your concerns with physical therapist or your surgeon.

Unlikely: There is no relationship between shoulder surgery and bowel movements unless you manually assist with your bm's. Otherwise, any shoulder pain from an abdominal process is referred pain from diaphragmatic irritation.

No relation: It is hard to imagine how these could be related. Nevertheless, if you are pushing so hard to have a bowel movement that you needed to ask this question, you should probably consider a stool softener like colace (docusate sodium) and adding fiber to your diet to avoid constipation.