How often should coolant flushes be done and how often for diff flush my blazer has 75k on is it due for all the flushes?

 

With a 2001 (found that through your other posts), you are over due for a coolant flush & refill if it has never been done. If your cooling system has maintained its integrity, then the coolant should be a simple drain, flush with a hose, then distilled water, and refill.

On the diffs, it won't hurt anything to drain and refill them. The rear diff requires a new gasket and while you're in there, you should clean it out well before filling it back up.