I am using the TFS 2010 Scrum template and want to modify the Task work item template so that I can display the PBI's backlog priority (read only).
I am familiar with editing the process templates, but not sure I can setup a field to pull data from another work item
There are no rules available to get data from other work items. If you want to do this, you can either use the eventing service (which is a service that is invoked on work item save for example) or you can create a custom work item control.