I wasn't able to get it working with @April Drake's solution as I was kept on getting the Due Date as None. Upon further search, I stumble upon this post and the steps in it helped to generated the email with the correct due date. Posting it here just for reference as an alternative to the accepted answer.
The Steps to get the email to include the custom due date;
1) Edit the Approval Action in your workflow
2)Select "Change the behavior of a single task"
3) In the "When a Task is Pending" step, find and select "Current Task:AssignedTo"
4) Replace %Variable: DueDateOnly% by %Current Task:Due Date%