i say work the cardiac unit. you need to have the monitor/ekg skills if you were to decide to end up in anesthesia. dialysis, although it often has a lot to do with diabetes, would not be the strongest background if you decide to do either of your listed options. i would say do cardiac for a year, use aides and other nurses to help you move your patients to avoid further back injury, learn your acls and organization skills (time management is huge), then work icu for a year if you decide to go back to school, as this will be the biggest asset to you for either career choices in the long run.
i'm a new grad, and that's what i would do if i were in your situation.
good luck!