I dug this out...
The front bumper height of trucks whose gross vehicle weight ratings are 4,500 pounds or less shall be no less than 14 inches and no more than 28 inches, and their rear bumper height shall be no less than 14 inches and no more than 28 inches. The front bumper height of trucks whose gross vehicle weight ratings are 4,501 pounds to 7,500 pounds shall be no less than 14 inches and no more than 29 inches, and their rear bumper height shall be no less than 14 inches and no more than 30 inches. The front bumper height of trucks whose gross vehicle weight ratings are 7,501 pounds to 15,000 pounds shall be no less than 14 inches and no more than 30 inches, and their rear bumper height shall be no less than 14 inches and no more than 31 inches.
No vehicle shall be operated on a public highway if it has been modified by any means so as to raise its body more than three inches, in addition to any manufacturer's spacers and bushings, above the vehicle's frame rail or manufacturer's attachment points on the frame rail.