Ask and ye shall receive:
Direct from the Utah Legislative Website-
Utah Code Section 41: Motor Vehicles
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;
(ii) any truck with a gross vehicle weight rating of 10,500 pounds or more;
(iii) any truck tractor; and
(iv) any trailer or semitrailer with an unladen weight of 750 pounds or more.
(b) The wheel covers, mudguards, flaps, or splash aprons shall:
(i) be at least as wide as the tires they are protecting;
(ii) be directly in line with the tires; and
(iii) have a ground clearance of not more than 50% of the diameter of a rear-axle wheel, under any conditions of loading of the motor vehicle.
(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; or
(b) on a vehicle operated or driven during fair weather on well-maintained, hard-surfaced roads if the motor vehicle:
(i) was made in America prior to 1935;
(ii) is registered as a vintage vehicle; or
(iii) is a custom vehicle as defined under Section 41-6a-1507.
(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.
Amended by Chapter 171, 2009 General Session
Interestingly, it looks like another code section makes wheel spacers illegal. I have no idea how strict they are about this stuff, especially in Moab for EJS which is a huge money maker for the city. With that said though, I could see some podunk towns outside of Moab using this week to fill their coffers.