Answer: Tell your manager about this offensive behavior.
Step-by-step explanation:
If I overhear a group of your co-workers laughing at some crude jokes about a few customers, the most likely thing for me to do will be to inform my manager about this offensive behavior.
Customers are vital to every business and should be treated right, without the customers, there isn't any business at all. Therefore, I'll inform my manager so that he'll have an idea of what is going on and then call them to order and explain to them that customers should be treated right and respected.