Each vehicle you look at will have a Max Tow Rating and possible a GCWR as well. That rating can vary a lot depending on the exact model chosen, e..g different engine, transmission, rear axle, etc. GM and Ford both publish towing guides for each years models, but Trailer Life magazine does one that covers most brands as well. Chrysler (Ram) should have a tow guide too, but I've only seen the digital look-up application on their website.
To get into the 6000 lb class, you probably need a full size SUV like the Tahoe/Suburban or Ford Expedition or E350 van or a pick-up truck (a crew cab model has plenty of seating if that is a requirement).
Here is a link to some Trailer Life towing guides that might help.
http://www.trailerlife.com/trailer-towing-guides/