Hi almustin1,
CareerKeysMan gave you great advice and you should be able to glean dates from a social security earnings report (this normally incurs a fee).
Are you thinking about omitting dates on your resume? I don't recommend that, as employers will be looking for dates and trying to understand your career chronology. Can you recall the years of employment? Your resume doesn't have to include months. If a hiring manager asks for months and you still don't have them, just be honest like you were here.
I wrote an article on recovering your resume -- this was written following a natural disaster, but there might be some ideas that will help you:
Resume Recovery TipsIt sounds like you've been doing consulting work over the last few years, correct? If so, include your own business and give yourself an appropriate title. In the description, focus on the tasks that would be important for your job target. For example, for marcomm, you can include marketing communications projects such as brochures and business card development. For admin goals, you can emphasize clerical tasks, project management, computer skills, etc. It's a bit time-consuming because you will need to customize your resume for each of your goals, but this should help minimize the perception that you're "overqualified."
I hope this helps and good luck to you!