I special ordered a 1993 Ford Crown Vic with the HD trailer towing package. It could tow 5,000 pounds. It required a weight distributing hitch, and had a maximum frontal area requirement for a trailer. It had a bigger radiator, transmission cooler, stronger ujoints, dual exhaust and a lower rear axle ratio. You had to be sure and turn off OD when towing. Towed my 5,000 pound boat to florida several times, and launched the boat almost every summer weekend.
EDIT: I forud this page of a brochure that describes the changes for the HD trailer towing package. You will have to enlarge the print to see it. It is in footnote 6.
http://www.lov2xlr8.no/brochures/ford/92vic_15.html