If your ex refuses to show you his Income Tax Return ( as was ordered by the Judge in the divorce decree) can he be taken to court using the " to show just cause law?". This is an Iowa case

You need to file a Motion for Contempt against him if this was ordered by the Judge. The Judge will rule on this and Order him to show you his income tax. If he does not do this then he would be in contempt of court and the Judge could put him in jail if he does not comply with the Judge's Order.

The show just cause law would not be what you need to bring to the Judge's attention. It would be a Motion for Contempt instead.