Even tho' your lighter frame is not ultra-light at 300 grams, it still could be generating quite a bit of frame shock if you are hitting somewhat hard (or playing against others who hit fairly hard). It is frame shock (not vibrations) that results in shoulder crunch and wrist crunch from repeated ball impacts.
Try the Racquets forum if you want more info on the inherent shock potential of your lighter frame. And try the Health & Fitness forum for more about exercises for the shoulder and forearm.
If u have one foot stuck in yesterday & one foot stuck in tomorrow, u are peeing on today.