Cold-setting a rear w/ a 2x4 isn't that difficult. You didn't mention if the dropout spacing itself, but assuming it's the original (not spread), then you'll simply have to be patient and work one side at a time. I'd move the off side 1mm or so, then work on moving the other side to follow it. 5mm isn't that much, it should come with a little patience and thoughtful application of force.