Which collections contain annual legislative priorities of various unions in the U.S. from WWII onward?
I would suggest looking for information coming out of union legislative departments/offices. The AFL and CIO (and later the AFL-CIO) each had a department, as does the UAW. We have publications and manuscript collections for each, descriptions of which are available on our website. You could also try contacting the collection archivists on staff here for AFSCME, AFT, SEIU, UAW, and UFW to get more detailed information about specific trades or time periods.