What is the difference between zhèng and zhēng in 挣?
I think you already got the point. 


  
  挣 zhēng: see 掙扎｜挣扎
  挣 zhèng: to struggle to get free / to strive to acquire / to make (money)
  


挣 will only be pronounced as 'zhēng' in 挣扎 and 'zhèng' for the rest.