Cooking defrosted then rechilled beef in a slow cooker
From the sounds of it: Probably, yes.

It's about total time spent in the 'danger zone' (40°F - 140°F,5°C - 58°C).  Total time shouldn't exceed two hours.  

According to your process, the only time should be been whatever cooling time happened between it being browned and placed back in the fridge.  Your slow cooker is going to bring it up to above that temp slow enough that there is a time there that will happen.  

So, on the very safe side,  if you slow cooker takes 1 hour to get up to that temp and you kept your cooked beef out to cool before going fridge for less than 1 hour (that would be 2 hours total) - then you're safe.