Just checked the u-haul web site for you. I don't see where the tow vehicle can not be a Ford Explorer.
From the site.... "
Towing vehicle requirements
Must be equipped with a hard top if it is an SUV. U-Haul does not allow any type of sport-utility vehicle not equipped with a hard top to tow U-Haul equipment.
Must weigh at least 3,500 lbs. (curb weight), and must equal or exceed 80% of the combined weight of the trailer (2,210 lbs.) and the vehicle being towed (up to 5,290 lbs.).
Must have a towing system that has a 5,000 lbs. minimum weight-carrying rating
Must have 1-7/8" or 2" hitch ball (5,000 lbs. minimum)
Must not exceed maximum allowable hitch ball height 25"
Must have external mirrors on both sides
Must have fully operational lighting
Auto transport lighting must be operational at all times, day and night, loaded or unloaded
Maximum recommended speed is 55 MPH
Load trailers heavier in front.
Reduce normal driving speed.
Wear a seatbelt."
I would like to see the policy they quoted you...