Its a shame that you dont have any more information on group code, the issue with Explorers is that they come in 5 and 7 passenger versions so Hertz tends to classify them as 4x4 category.

If you absolutely do need 7 passenger then you should look at category that includes "minibus" they tend to offer a 7 seat Toyota or a larger Ford van.

Best thing to do is just phone them, explain the problem and see what they say. I am sure that they will help you out.