It can be hard to find a great minivan rental deal. They are a high demand vehicle during the summer and during all weekends and holidays. I don't think that very many
companies run specials on minivans very regularly at all. In fact, the only one that I currently (June 2009) know of is through Thrifty....
They are currently running minivan specials with Travelocity for $39.99 a day or $239 a week at participating locations for pick up until June 20, 2009.
Otherwise, I would continue to search, as you are already doing.
Make sure to try your online coupon codes, (you can also try searching for more Free Rental Car Coupon Codes and book as far in advance as you can! You may be able to find a better rate if you do a PrePaid Rental in advance. And never forget to check to see if you are a member of any association that offers Discount Codes to their members.
Good Luck with your search, I hope you find a minivan rental deal soon!