I haven't seen all the other threads but from what you posted I can't figure out why you are using DATEDIFF at all. It doesn't meet your requirements and it will make your query nonSARGable.
I think that something like this should meet your requirements. Of course as Lynn stated this doesn't accomodate NULL values in any of those columns.
[WorkForce_JobPosting].[Job Posting End Date_JP] < [WorkForce_WorkOrder].[Work Order End Date_WO]
AND
[WorkForce_WorkOrder].[Previous Work Order End Date_WO] <= [WorkForce_WorkOrder].[Work Order End Date_WO]
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