There is a User Define Contact Expert on the PracticeWorks Expert Exchange that should give you what you're looking for. It can be downloaded by following the steps below:
1.) Go to the Experts Menu>PracticeWorks Expert Exchange (this may require a level 10 access depending on how your security is set up).
2.) When the Expert Exchange window comes up, scroll down until you see the second listing for PWSupport and click the plus sign to the left of it to expand.
3.) Scroll down until you see Uninsured Patients by Last Visit and put a check mark in the box to the left.
4.) Remove the check mark from "Install to Tutorial Database."
5.) Click on Download.
6.) Click Accept.
7.) Click Accept.
8.) Click Ok.
9.) Click Close.
10.) Click Close.
After the expert is installed, go to Experts>Contact Expert>User Defined and find this new Uninsured Patients by Last Visit expert and run it. Is should display uninsured patients who were last seen within the last visit date range you set.