Shoulder problems often refer to technique faults in serve. Have a teaching pro look at it.
For me it's a continental grip. And hit the ball when it's inside the court above your head. Then it's like you're throwing your racquet at receivers feet. Throwing motion should be very loose and should be practiced without a ball.
Born 1978. Male 4.5 50/50 competetive/recreational player. Gut/poly.