Isn't the Tour 90 quite a bit stiffer than the i.Prestige?
Stick with a flexible racquet and use soft string. Multifilament of Nat Gut.
It can also be your serving mechanics that are causing the problem. My shoulder is also sensitive after dislocating it about a year ago. I find that it mostly hurts if I'm not hitting the ball in the right spot. Too far back, too low, and too far to the right are probably the biggest pain causing tosses for me to go after.