The compare module comes with WeBWorK. It enables an instructor to view two pg problems. It renders both problems, and gives "diffs" of the source. If you have the program "hdiff" on your source, you can get a colored diff of the source files (this requires uncommenting some lines in Compare.pm).
To enable the compare module on your server, edit the file webwork2/lib/WeBWorK/ContentGenerator.pm. Search for Compare; it appears in one commented line. Uncomment that line, and restart your server.