Those stiff rackets would worry me.
LeeD has a good idea, though some people (like me) don't serve well with oversized rackets.
I'm thinking the opposite of many here. Perhaps the extended racket forced you to have a longer, more fluid motion. You may be forcing the racket around trying to repeat the power you were getting with the extended racket. (Due to the longer distance to the sweetspot, you will naturally get more serve power with an extended racket.)
As an experiment, put some lead at the top of the frame and practice a relaxed, long, smooth, slowly accelerating service motion - not worrying about power initially. You may find when you get your timing down that you get a lot more power with a lot less effort while saving your shoulder.
If the experiment works, then look at how much lead you want to keep, taking into account the rest of your game. You may not need any once your motion is smoothed out.