I have have towed U-Haul trailers down the road at 75 before with no issues. It's an insurance thing for them. Double check their tires though, they are usually crap. Secure the load and distribute it properly and you shouldn't have any issues.
However.....do this at your own risk!!! I am not responsible if you have a crack-up because you were towing a U-Haul trailer to fast.
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08 Double Cab SR5
5.7 4x4 TRD
Pyrite Mica
IBTO
Mods:
Tinted Windows
Hurst Shifter
Kobalt Narrow Over Rail Tool Box
Under Seat Storage Box
Toyota Bed Mat
HD Trailer Brake Conroller
G-Tek Door Sill Protectors
International Brotherhood of Tundra Owners Headquarters