Basically, if you lift it or widen the wheelbase you are required to cover the width of the tires from 50% up at the rear wheels. You can do it with fenders, mud flaps or body panels.
From the UT code
41-6a-1633. Mudguards or flaps at rear wheels of trucks, trailers, truck tractors, or altered motor vehicles -- Exemptions.
(1) (a) Except as provided in Subsection (2), when operated on a highway, the following vehicles shall be equipped with wheel covers, mudguards, flaps, or splash aprons behind the rearmost wheels to prevent, as far as practicable, the wheels from throwing dirt, water, or other materials on other vehicles:
(i) a vehicle that has been altered:
(A) from the original manufacturer's frame height; or
(B) in any other manner so that the motor vehicle's wheels may throw dirt, water, or other materials on other vehicles;
(2) Wheel covers, mudguards, flaps, or splash aprons are not required:
(a) if the motor vehicle, trailer, or semitrailer is designed and constructed so that the requirements of Subsection (1) are accomplished by means of fenders, body construction, or other means of enclosure;
(3) Except as provided in Subsection (2)(b), rear wheels not covered at the top by fenders, bodies, or other parts of the vehicle shall be covered at the top by protective means extending rearward at least to the center line of the rearmost axle.
